CONDENSED CONSOLIDATING FINANCIAL INFORMATION (Schedule of Condensed Consolidating Statements of Cash Flows) (Details) - USD ($) $ in Millions			3 Months Ended		9 Months Ended		12 Months Ended
	Feb. 26, 2018	Jul. 05, 2017	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018	Jun. 30, 2018	Jun. 30, 2017
Net cash provided by operating activities					$ 726.0	$ 719.5
Cash flows from investing activities:
Purchases of property and equipment			$ (206.4)	$ (195.1)	(591.1)	(581.9)
Acquisitions, net of cash acquired						(155.3)
Proceeds from sale of SRT, net of cash held in escrow					39.0
Other						(0.2)
Net cash used in investing activities					(552.1)	(737.4)
Cash flows from financing activities:
Proceeds from debt	$ 150.0	$ 310.7			275.0	462.8
Principal payments on long-term debt					(108.8)	(314.4)
Principal repayments on capital lease obligations					(5.6)	(6.4)
Payment of debt issuance costs						(4.2)
Payments to repurchase ZGH common stock					(402.5)
Cash paid for Santa Clara acquisition					(5.8)	(3.8)
Net cash (used in)/provided by financing activities					(247.7)	134.0
Net cash flows					(73.8)	116.1
Effect of changes in foreign exchange rates on cash					(6.6)	(7.3)
Net (decrease)/increase in cash, cash equivalents and restricted cash					(80.4)	108.8	$ (80.4)	$ 108.8
Cash, cash equivalents and restricted cash, beginning of year					260.6	224.5	224.5
Cash, cash equivalents and restricted cash, end of period			180.2	333.3	180.2	333.3	260.6	224.5
Zayo Group, LLC [Member]
Net cash provided by operating activities					628.0	600.6
Cash flows from investing activities:
Purchases of property and equipment					(486.3)	(531.1)
Acquisitions, net of cash acquired						(130.5)
Proceeds from sale of SRT, net of cash held in escrow					39.0
Other					(10.4)	(9.9)
Net cash used in investing activities					(457.7)	(671.5)
Cash flows from financing activities:
Proceeds from debt					275.0	462.8
Principal payments on long-term debt					(108.8)	(314.4)
Principal repayments on capital lease obligations					(4.6)	(5.2)
Payment of debt issuance costs						(5.1)
Payments to repurchase ZGH common stock					(402.5)
Contributions to parent					(13.7)	(1.9)
Cash paid for Santa Clara acquisition					(5.8)	(3.8)
Net cash (used in)/provided by financing activities					(233.0)	136.2
Net cash flows						65.3
Net (decrease)/increase in cash, cash equivalents and restricted cash							(62.7)	65.3
Cash, cash equivalents and restricted cash, beginning of year					142.7	100.0	100.0
Cash, cash equivalents and restricted cash, end of period			80.0	165.3	80.0	165.3	142.7	100.0
Guarantor Subsidiaries [Member]
Net cash provided by operating activities					3.7	4.4
Cash flows from financing activities:
Contributions to parent					3.4	1.9
Net cash (used in)/provided by financing activities					(3.4)	(1.9)
Net cash flows						2.5
Net (decrease)/increase in cash, cash equivalents and restricted cash							0.3	2.5
Cash, cash equivalents and restricted cash, beginning of year					5.9	2.1	2.1
Cash, cash equivalents and restricted cash, end of period			6.2	4.6	6.2	4.6	5.9	2.1
Non-Guarantor Subsidiaries [Member]
Net cash provided by operating activities					94.3	114.5
Cash flows from investing activities:
Purchases of property and equipment					(104.8)	(50.8)
Acquisitions, net of cash acquired						(24.8)
Other					10.4	9.7
Net cash used in investing activities					(94.4)	(65.9)
Cash flows from financing activities:
Principal repayments on capital lease obligations					(1.0)	(1.2)
Payment of debt issuance costs						0.9
Contributions to parent					10.3
Net cash (used in)/provided by financing activities					(11.3)	(0.3)
Net cash flows						48.3
Effect of changes in foreign exchange rates on cash					(6.6)	(7.3)
Net (decrease)/increase in cash, cash equivalents and restricted cash							(18.0)	41.0
Cash, cash equivalents and restricted cash, beginning of year					112.0	122.4	122.4
Cash, cash equivalents and restricted cash, end of period			$ 94.0	$ 163.4	$ 94.0	$ 163.4	$ 112.0	$ 122.4

ZAYO GROUP LLC 10-Q