Equity and Other Comprehensive Income (Loss) - Summary of Reclassification out of Accumulated Other Comprehensive Loss (Details) - USD ($)
$ in Millions

3 Months Ended

9 Months Ended

Jun. 30, 2018

Jun. 30, 2017

Jun. 30, 2018

Jun. 30, 2017

Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]

Amortization and settlement recognition of net actuarial loss, included in pension cost

$ 5.2

$ 3.4

$ 11.8

$ 30.2

Amortization of prior service (cost) credit, Net of Tax

0.2

(0.1)

0.2

(0.4)

Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale of HH&B, Net of Tax

0.0

26.8

0.0

26.8

Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax

0.0

(0.1)

0.5

(0.1)

Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, Sale of HH&B, Net of Tax

0.0

2.9

0.0

2.9

Available for sale security, Net of Tax

0.0

(1.5)

0.0

Parent [Member]

Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]

Amortization of net actuarial loss, Pre-Tax Amount

^[1],[2]

(6.8)

(4.8)

(15.7)

(46.7)

Amortization of net actuarial loss, Tax

^[1],[2]

1.9

1.5

4.3

16.9

Amortization and settlement recognition of net actuarial loss, included in pension cost

^[1],[2]

(4.9)

(3.3)

(11.4)

(29.8)

Amortization of prior service (cost) credit, Pre-Tax Amount

^[1]

(0.3)

^[2]

0.2

^[2]

(0.3)

0.7

Amortization of prior service (cost) credit, Tax

^[1]

0.1

^[2]

(0.1)

^[2]

0.1

(0.3)

Amortization of prior service (cost) credit, Net of Tax

^[1]

(0.2)

^[2]

0.1

^[2]

(0.2)

0.4

Defined Benefit Plans, before Tax

^[1]

(7.1)

(8.8)

(16.0)

(50.2)

Defined Benefit Plans, Tax

^[1]

2.0

2.7

4.4

17.9

Defined Benefit Plans, Net of Tax

^[1]

(5.1)

(6.1)

(11.6)

(32.3)

Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale of HH&B, before Tax

^[1]

0.0

^[3]

(26.8)

^[3]

0.0

^[4]

(26.8)

^[4]

Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale of HH&B, Tax

^[1]

0.0

^[3]

0.0

^[3]

0.0

^[4]

0.0

^[4]

Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale of HH&B, Net of Tax

^[1]

0.0

^[3]

(26.8)

^[3]

0.0

^[4]

(26.8)

^[4]

Total Reclassifications From Other Comprehensive Income Before Tax

^[1]

(7.1)

(35.3)

(15.2)

(76.8)

Total Reclassifications From Other Comprehensive Income Tax Portion

^[1]

2.0

2.5

4.6

17.8

Total Reclassifications From Other Comprehensive Income Net Of Tax

^[1]

(5.1)

(32.8)

(10.6)

(59.0)

Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, Sale of HH&B, before Tax

^[1]

0.0

^[3]

(4.2)

^[3]

0.0

^[4]

(4.2)

^[4]

Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, Sale of HH&B, Tax

^[1]

0.0

^[3]

1.3

^[3]

0.0

^[4]

1.3

^[4]

Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, Sale of HH&B, Net of Tax

^[1]

0.0

^[3]

(2.9)

^[3]

0.0

^[4]

(2.9)

^[4]

Available for sale security, before Tax

^[1],[5]

1.5

0.0

Available for sale security, Tax

^[1],[5]

0.0

Available for sale security, Net of Tax

^[1],[5]

1.5

0.0

Parent [Member] | Foreign Exchange Contract [Member]

Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]

Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax

^[1],[6]

0.0

0.3

(0.7)

0.2

Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax

^[1],[6]

0.0

(0.2)

0.2

(0.1)

Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax

^[1],[6]

$ 0.0

$ 0.1

$ (0.5)

$ 0.1

[1]	Amounts in parentheses indicate charges to earnings. Amounts pertaining to noncontrolling interests are excluded.
[2]	Included in the computation of net periodic pension cost. See “Note 4. Retirement Plans” for additional details.
[3]	Included in gain on sale of HH&B.
[4]	Included in gain on sale of HH&B
[5]	Included in other income, net.
[6]	Included in net sales.

WestRock Co 10-Q