Income tax (Details) - USD ($)	3 Months Ended		12 Months Ended
Income tax (Details) - USD ($)	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred Tax Benefit on Goodwill Impairment				$ 51,100,000
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount				77,800,000
Reductions due to a lapse of the statute of limitations	$ 7,500,000	$ 3,700,000	$ (11,800,000)	(4,500,000)	$ 1,000,000
Current Income Tax Expense (Benefit), Continuing Operations [Abstract]
U.S. federal			10,200,000	68,100,000	63,200,000
Foreign			22,300,000	16,200,000	20,100,000
State and local			2,000,000	5,600,000	5,200,000
Total current tax expense			34,500,000	89,900,000	88,500,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Total deferred tax expense			25,500,000	(131,200,000)	26,200,000
Provision for income taxes			60,000,000	(41,300,000)	114,700,000
Net deferred tax liabilities			129,500,000	102,000,000
Tax impact of Brazil court decision	5,200,000.0		5,200,000.0
Effect of Tax Cuts and Jobs Act of 2017, Repatriation Transition Tax				10,600,000
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense			400,000	1,100,000	1,200,000
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest [Abstract]
U.S. income before income taxes			201,700,000	210,900,000	283,600,000
Non-U.S. income before income taxes			84,000,000	(158,200,000)	67,300,000
Income from continuing operations before income taxes			285,700,000	52,700,000	350,900,000
Components of Deferred Tax Assets [Abstract]
Accounts receivable, principally due to allowances for returns and doubtful accounts			2,800,000	2,900,000
Inventories, principally due to additional costs inventories for tax purposes			14,000,000	17,700,000
Employee compensation and benefits accrued for financial reporting purposes			45,500,000	36,500,000
Foreign net operating losses			19,400,000	24,500,000
Foreign tax credits			15,100,000	34,100,000
Other			13,200,000	16,100,000
Total deferred tax assets			110,000,000	131,800,000
Less valuation allowance			(33,100,000)	(48,500,000)
Total deferred tax assets after valuation allowance			76,900,000	83,300,000
Components of Deferred Tax Liabilities [Abstract]
Plant and equipment, principally due to differences in depreciation, capitalized interest, and capitalized overhead			89,500,000	86,400,000
Goodwill and intangible assets, principally due to differences in amortization			103,800,000	98,900,000
Deferred Tax Liabilities, Prepaid Expenses			13,100,000	0
Total deferred tax liabilities			206,400,000	185,300,000
Deferred tax liabilities, net			129,500,000	102,000,000
Deferred Tax Assets, Net, Classification [Abstract]
Deferred tax liabilities			166,700,000	153,500,000
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Computed expected tax expense on income before taxes at federal statutory rate			60,000,000	18,500,000	122,800,000
Increase (Decrease) Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
State and local income taxes net of federal income tax benefit			3,800,000	3,300,000	5,600,000
Foreign tax rate differential			5,800,000	(4,700,000)	(7,400,000)
Manufacturing tax benefits			0	(6,000,000)	(5,800,000)
Other			500,000	1,700,000	500,000
Actual income tax expense			$ 60,000,000	$ (41,300,000)	$ 114,700,000
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]
Federal statutory rate			21.00%	35.00%	35.00%
Increase (Decrease) Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]
State and local income taxes net of federal income tax benefit			1.40%	6.20%	1.60%
Foreign tax rate differential			2.00%	(8.90%)	(2.10%)
Manufacturing tax benefits			(0.00%)	(11.30%)	(1.70%)
Other			0.20%	3.10%	0.20%
Effective tax rate			21.00%	(78.40%)	32.70%
Remeasurement period benefit	0
Overall transition tax	0
transition tax obligation	0
transition tax obligation net benefit	0
Effective Income Tax Rate Reconciliation, Goodwill Impairment Rate Differential, Amount			$ 0	$ 17,700,000	$ 0
Effective Income Tax Rate Reconciliation, Goodwill Impairment Rate Differential, Percent			0.00%	33.60%	0.00%
Effective Income Tax Rate Reconciliation, Effect of Tax Cuts and Jobs Act of 2017, Amount			$ (8,200,000)	$ 67,200,000	$ 0
Effective Income Tax Rate Reconciliation, Effect of Tax Cuts and Jobs Act of 2017, Percent			(2.90%)	(127.50%)	0.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Compensation Cost, Amount			$ 400,000	$ (900,000)	$ 0
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Compensation Cost, Percent			0.10%	(1.70%)	0.00%
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of year	35,200,000	32,900,000	$ 35,200,000	$ 32,900,000
Additions based on tax positions related to the current year			3,100,000	3,900,000
Additions for tax positions of prior years			2,000,000	4,100,000
Reductions for tax positions of prior years			(3,000,000)	(1,100,000)
Settlements			(400,000)	(100,000)
Balance at end of year			25,100,000	35,200,000	$ 32,900,000
Operating loss carryforwards			71,000,000
Operating loss carryforwards subject to expiration			12,200,000
Foreign tax credit carryforward			15,100,000
Valuation allowance			33,100,000	48,500,000
Undistributed foreign subsidiaries earnings			293,800,000
Unrecognized tax benefits that would impact effective tax rate			25,100,000	35,200,000
Interest and penalties			11,500,000	14,500,000
Unrecognized Tax Benefits, Increase Resulting from Acquisition					2,600,000
Interest and penalties resulting from acquisitions					2,500,000
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit			6,000,000
Continuing Operations [Member]
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
U.S. federal			14,700,000	(73,200,000)	26,700,000
Foreign			6,900,000	(58,500,000)	(3,900,000)
State and local			3,900,000	500,000	3,400,000
Total deferred tax expense			25,500,000	(131,200,000)	26,200,000
Deferred charges and other assets [Member]
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Net deferred tax liabilities			37,200,000	51,500,000
Components of Deferred Tax Liabilities [Abstract]
Deferred tax liabilities, net			$ 37,200,000	$ 51,500,000
Statute closing percent [Domain]
Reductions due to a lapse of the statute of limitations	$ 0.026	$ 0.069			$ 0.003

BEMIS CO INC 10-K