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Debt: - Net Gain (Loss) on Extinguishment of Debt (Details) - USD ($) $ in Millions		1 Months Ended	3 Months Ended	12 Months Ended
	Aug. 02, 2018	Dec. 31, 2017	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Extinguishment of Debt [Line Items]
Discount (Premium) on early redemption					$ 226.0	$ (62.9)
Third-party fees for early redemption					0.0	(2.0)
Unamortized (discount) premium on original issuance, net					(22.9)	17.0
Unamortized debt issuance costs on original issuance					(12.8)	(8.5)
Net gain (loss) on early extinguishment of debt	$ 190.3		$ 190.3	$ 0.0	190.3	(56.4)
Unsecured Debt \| Exchanges of 2021, 2022, April 2023, August 2023 and 2024 Notes
Extinguishment of Debt [Line Items]
Discount (Premium) on early redemption					226.0
Third-party fees for early redemption					0.0
Unamortized (discount) premium on original issuance, net					(22.9)
Unamortized debt issuance costs on original issuance					(12.8)
Net gain (loss) on early extinguishment of debt		$ (55.5)			$ 190.3
Unsecured Debt \| Partial repurchase of 2020 Notes
Extinguishment of Debt [Line Items]
Discount (Premium) on early redemption						5.3
Third-party fees for early redemption						0.0
Unamortized (discount) premium on original issuance, net						0.1
Unamortized debt issuance costs on original issuance						(0.4)
Net gain (loss) on early extinguishment of debt						5.0
Unsecured Debt \| Exchanges of 2020, 2021, 2022, and April 2023 Notes
Extinguishment of Debt [Line Items]
Discount (Premium) on early redemption						(49.9)
Third-party fees for early redemption						(2.0)
Unamortized (discount) premium on original issuance, net						2.2
Unamortized debt issuance costs on original issuance						(5.8)
Net gain (loss) on early extinguishment of debt						(55.5)
Secured Debt \| Senior secured credit facility
Extinguishment of Debt [Line Items]
Discount (Premium) on early redemption						0.0
Third-party fees for early redemption						0.0
Unamortized (discount) premium on original issuance, net						(1.8)
Unamortized debt issuance costs on original issuance						(2.3)
Net gain (loss) on early extinguishment of debt						(4.1)
Broadview measurement period \| Secured Debt
Extinguishment of Debt [Line Items]
Discount (Premium) on early redemption						0.0
Third-party fees for early redemption						0.0
Unamortized (discount) premium on original issuance, net						0.2
Unamortized debt issuance costs on original issuance						0.0
Net gain (loss) on early extinguishment of debt						0.2
EarthLink \| Secured Debt
Extinguishment of Debt [Line Items]
Discount (Premium) on early redemption						(18.3)
Third-party fees for early redemption						0.0
Unamortized (discount) premium on original issuance, net						16.3
Unamortized debt issuance costs on original issuance						0.0
Net gain (loss) on early extinguishment of debt						$ (2.0)