CONDENSED CONSOLIDATING FINANCIAL INFORMATION (Schedule of Condensed Consolidating Statements of Cash Flows) (Details) - USD ($) $ in Millions	3 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Net cash provided by operating activities	$ 311.8	$ 241.8
Cash flows from investing activities:
Purchases of property and equipment	(217.1)	(182.5)
Proceeds from sale of SRT, net of cash held in escrow		39.0
Net cash used in investing activities	(217.1)	(143.5)
Cash flows from financing activities:
Principal payments on long-term debt	(21.3)	(1.3)
Principal repayments on finance lease obligations	(2.1)	(1.9)
Payments to repurchase ZGH common stock		(0.2)
Cash paid for Santa Clara acquisition		(3.3)
Net cash used in financing activities	(23.4)	(6.7)
Effect of changes in foreign exchange rates on cash	(2.7)	2.2
Net increase in cash, cash equivalents and restricted cash	68.6	93.8
Cash, cash equivalents and restricted cash, beginning of period	186.6	260.6
Cash, cash equivalents and restricted cash, end of period	255.2	354.4
Zayo Group, LLC [Member]
Net cash provided by operating activities	277.2	212.0
Cash flows from investing activities:
Purchases of property and equipment	(184.3)	(150.5)
Proceeds from sale of SRT, net of cash held in escrow		39.0
Net cash used in investing activities	(184.3)	(111.5)
Cash flows from financing activities:
Principal payments on long-term debt	(21.3)	(1.3)
Principal repayments on finance lease obligations	(2.0)	(1.6)
Payments to repurchase ZGH common stock		(0.2)
Contributions to parent	(9.5)	(8.7)
Cash paid for Santa Clara acquisition		(3.3)
Net cash used in financing activities	(32.8)	(15.1)
Net increase in cash, cash equivalents and restricted cash	60.1	85.4
Cash, cash equivalents and restricted cash, beginning of period	86.3	142.7
Cash, cash equivalents and restricted cash, end of period	146.4	228.1
Guarantor Subsidiaries [Member]
Net cash provided by operating activities	1.4	3.1
Cash flows from financing activities:
Contributions to parent	(3.6)	(1.6)
Net cash used in financing activities	(3.6)	(1.6)
Net increase in cash, cash equivalents and restricted cash	(2.2)	1.5
Cash, cash equivalents and restricted cash, beginning of period	9.6	5.9
Cash, cash equivalents and restricted cash, end of period	7.4	7.4
Non-Guarantor Subsidiaries [Member]
Net cash provided by operating activities	33.2	26.7
Cash flows from investing activities:
Purchases of property and equipment	(32.8)	(32.0)
Net cash used in investing activities	(32.8)	(32.0)
Cash flows from financing activities:
Principal repayments on finance lease obligations	(0.1)	(0.3)
Contributions to parent	13.1	10.3
Net cash used in financing activities	13.0	10.0
Effect of changes in foreign exchange rates on cash	(2.7)	2.2
Net increase in cash, cash equivalents and restricted cash	10.7	6.9
Cash, cash equivalents and restricted cash, beginning of period	90.7	112.0
Cash, cash equivalents and restricted cash, end of period	$ 101.4	$ 118.9