Long-Term Debt (Narrative) (Details) - USD ($) $ in Millions										1 Months Ended	3 Months Ended
	Jul. 20, 2017	Jul. 05, 2017	Apr. 10, 2017	Jan. 27, 2017	Jan. 19, 2017	Jul. 22, 2016	Apr. 14, 2016	Jan. 15, 2016	May 06, 2015	Apr. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2017	Jan. 19, 2016	Mar. 09, 2015	Jan. 23, 2015
Debt Instrument [Line Items]
Payments of Debt Extinguishment Costs	$ 4.9
Debt obligations											$ 5,596.8		$ 5,608.7
Proceeds from issuance of private placement		$ 310.7									312.8
Repayment of debt											311.9
Loss on extinguishment of debt											$ 4.9
Secured debt ratio											4.50
Debt issuance costs											$ 113.1
Accumulated amortization											47.4	$ 45.1
Interest expense associated with the amortization of debt issuance costs											$ 2.3	$ 2.2
Revolver [Member]
Debt Instrument [Line Items]
Weighted average interest rate											4.00%		3.80%
Letters of Credit [Member]
Debt Instrument [Line Items]
Line of credit facility maximum borrowing capacity									$ 50.0
Unused commitment, percentage									0.25%
Secured Debt [Member] | Revolver [Member]
Debt Instrument [Line Items]
Line of credit facility maximum borrowing capacity									$ 450.0
Remaining borrowing capacity											$ 442.2
Percentage of excess revolver committed to debt payments											35.00%
Secured debt ratio											5.25
Secured Debt [Member] | Letters of Credit [Member]
Debt Instrument [Line Items]
Outstanding letters of credit											$ 7.8
Term Loan Facility
Debt Instrument [Line Items]
Payment towards principal									$ 5.1
Percentage of excess cash flows committed to debt payments									50.00%
Discount on debt											$ 13.6		$ 16.0
Weighted average interest rate											3.40%		3.40%
Aggregate principal amount outstanding											$ 1,616.8
Debt obligations					$ 2,500.0
Repriced percentage					99.75%
Repayment of debt			$ 570.1				$ 196.0			$ 310.7
Incremental Term Loan [Member]
Debt Instrument [Line Items]
Line of credit facility increase to borrowing capacity								$ 400.0
Priced percent of additional line of credit								99.00%
Debt obligations					$ 1,850.0
Repriced percentage					99.75%
Incremental Term Loan [Member] | Electric Lightwave
Debt Instrument [Line Items]
Interest rate decrease (basis point)	(0.25%)
Debt obligations					$ 650.0
Minimum | Revolver [Member]
Debt Instrument [Line Items]
Unused commitment, percentage									0.25%
Maximum
Debt Instrument [Line Items]
Total indebtedness ratio											6.00
Maximum | Revolver [Member]
Debt Instrument [Line Items]
Unused commitment, percentage									0.375%
Tranche One [Member] | Incremental Term Loan [Member]
Debt Instrument [Line Items]
Interest rate decrease (basis point)					(0.75%)
Debt obligations														$ 500.0
Tranche Two [Member] | Incremental Term Loan [Member]
Debt Instrument [Line Items]
Interest rate decrease (basis point)					(0.25%)
Debt obligations					$ 1,350.0
6.00% Senior Notes due 2023 [Member]
Debt Instrument [Line Items]
Debt obligations											$ 1,430.0		$ 1,430.0
Interest rate of debt redeemed											6.00%
6.00% Senior Notes due 2023 [Member] | Unsecured Notes | Zayo Group Ltd (ZGL) [Member]
Debt Instrument [Line Items]
Debt obligations																$ 700.0
6.00% Senior Notes due 2023 [Member] | Unsecured Notes | Zayo Capital [Member]
Debt Instrument [Line Items]
Debt obligations															$ 730.0
6.375% New Senior Notes due 2025 [Member] | Unsecured Notes | Zayo Group Ltd (ZGL) and Zayo Capital [Member]
Debt Instrument [Line Items]
Debt obligations							$ 550.0
6.375% Senior Notes due 2025 [Member]
Debt Instrument [Line Items]
Discount on debt											$ 13.9		14.3
Debt obligations											$ 900.0		900.0
Debt price percentage							97.76%
Interest rate of debt redeemed											6.375%
6.375% Senior Notes due 2025 [Member] | Unsecured Notes | Zayo Group Ltd (ZGL) and Zayo Capital [Member]
Debt Instrument [Line Items]
Debt obligations							$ 350.0
5.75% Senior Unsecured Notes due 2027 [Member]
Debt Instrument [Line Items]
Debt obligations		$ 300.0	$ 550.0	$ 800.0							$ 1,650.0		$ 1,350.0
Debt price percentage		104.25%		104.00%
Interest rate of debt redeemed											5.75%
10.125% Senior Notes due 2020 [Member]
Debt Instrument [Line Items]
Repayment of debt							325.6
Redemption premium and accrued interest included in redemption amount							$ 20.3
Interest rate of debt redeemed							10.125%
LIBOR | Term Loan Facility
Debt Instrument [Line Items]
Interest rate decrease (basis point)									(0.25%)
Spread on interest rate									2.75%
LIBOR | Incremental Term Loan [Member]
Debt Instrument [Line Items]
Interest rate decrease (basis point)						(0.75%)
Spread on interest rate						2.75%		3.50%
LIBOR | Incremental Term Loan [Member] | Electric Lightwave
Debt Instrument [Line Items]
Spread on interest rate	2.25%				2.50%
LIBOR | Minimum | Letters of Credit [Member]
Debt Instrument [Line Items]
Spread on interest rate									2.00%
LIBOR | Minimum | Term Loan Facility
Debt Instrument [Line Items]
Spread on interest rate									1.00%
LIBOR | Minimum | Incremental Term Loan [Member]
Debt Instrument [Line Items]
Spread on interest rate						1.00%		1.00%
LIBOR | Minimum | Incremental Term Loan [Member] | Electric Lightwave
Debt Instrument [Line Items]
Spread on interest rate	1.00%				1.00%
LIBOR | Maximum | Letters of Credit [Member]
Debt Instrument [Line Items]
Spread on interest rate									3.00%
LIBOR | Tranche One [Member] | Incremental Term Loan [Member]
Debt Instrument [Line Items]
Spread on interest rate					2.00%
LIBOR | Tranche One [Member] | Minimum | Incremental Term Loan [Member]
Debt Instrument [Line Items]
Spread on interest rate					0.00%
LIBOR | Tranche Two [Member] | Incremental Term Loan [Member]
Debt Instrument [Line Items]
Spread on interest rate					2.50%
LIBOR | Tranche Two [Member] | Minimum | Incremental Term Loan [Member]
Debt Instrument [Line Items]
Spread on interest rate					1.00%