Debt: - Interest Expense (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Interest Expense, Net Disclosure [Line Items]
Interest expense - long-term debt		$ 338.7	$ 429.0	$ 376.1
Impact of interest rate swaps		(12.2)	(3.5)	10.1
Interest on capital leases and other		5.8	6.3	5.1
Less capitalized interest expense		(6.6)	(3.7)	(7.0)
Total interest expense	$ 13.8	331.9	901.3	875.4
Telecommunications network assets (a)
Interest Expense, Net Disclosure [Line Items]
Interest expense related to long-term lease obligations		0.0	467.0	484.9
Real estate contributed to pension plan
Interest Expense, Net Disclosure [Line Items]
Interest expense related to long-term lease obligations		$ 6.2	$ 6.2	$ 6.2