Description of Business and Accounting Policies Description of Business and Accounting Policies (Details)	3 Months Ended					12 Months Ended
	Sep. 30, 2017 USD ($)	Mar. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($)	Oct. 05, 2016 USD ($) shares	Oct. 04, 2016 shares	Jan. 24, 2013 shares
Description of accounting policies [Line Items]
Entity Number of Employees						3,500
Company Employees Participating in Collective Bargaining Agreement, Percentage						25.00%
Common Stock, Shares, Issued | shares			42,056,237			42,197,965	42,056,237			42,055,001	210,275,005
Total Par Value, Amount, reclassified										$ 1,700,000
Proceeds from sale of wireless spectrum licenses									$ 194,400,000
Spectrum, Cost to Use									$ 8.00
Cash and cash equivalents			$ 9,700,000			$ 17,800,000	$ 9,700,000
Restricted Cash, Current			$ 0			$ 378,700,000	$ 0
Trade Receivables Due From Customers, Range, Minimum						21
Trade Receivables Due from Customers, Range, Maximum						90
Number of customers, exceeds 10% of total accounts receivable			1			1	1
Accounts Receivable from one customer greater than 10%, percentage			21.00%			10.00%	21.00%
Unbilled Receivables, Current			$ 14,500,000			$ 14,200,000	$ 14,500,000
Factoring Fees						500,000
Accounts Receivable Sold						92,100,000
Investment owned in CyrusOne Inc., Balance, Shares | shares												1,900,000
Investment owned in CyrusOne Inc., Percentage, Common Stock												8.60%
Investment owned in Equity Method Investee, Balance, Partnership Units | shares								6,300,000				42,600,000
Investment owned in CyrusOne LP, Percentage, Partnership Units												66.00%
Total ownership interests in CyrusOne, Percentage												69.00%
Available for sale securities, ownership percentage								9.50%
Investment owned in Available for Sale Securities, Balance, Shares | shares			2,800,000				2,800,000	6,900,000
Other (income) expense, dividend income							$ 6,400,000
Available-for-sale Securities			$ 128,000,000				128,000,000
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, before Tax							$ 105,000,000
Available-for-Sale Securities, Shares Sold during the Period | shares		2,800,000					4,100,000
Proceeds from Sale of Available-for-sale Securities, Equity						140,700,000	$ 189,700,000	$ 643,900,000
Marketable Securities, Realized Gain (Loss)		$ 117,700,000	5,100,000	$ 33,300,000	$ 118,600,000	117,700,000	157,000,000	$ 449,200,000
Equity Method Investment, Other than Temporary Impairment	$ 4,700,000
Cost Method Investments			$ 3,400,000			$ 3,800,000	$ 3,400,000
Number of customers, exceeds 10% of total revenue							1	1
Revenue from one customer greater than 10%, percentage							12.00%	12.00%
Percent of Revenue from Foreign Subsidiaries						3.00%
Customer Contract, Lower Range, in Years						1
Customer Contract, Upper Range, in Years						3
Advertising Expense						$ 13,500,000	$ 9,500,000	$ 8,300,000
Regulatory Taxes Included in Revenue						16,800,000	16,300,000	15,500,000
Regulatory Taxes Included in Expense						$ 17,700,000	$ 17,500,000	$ 17,900,000
Lower range, in years, of remaining service life of active employees						8
Upper range, in years, of remaining service life of active employees						12
Other Postretirement Benefits Plan
Description of accounting policies [Line Items]
Average life expectancy of retirees, in years						15
8% Senior Notes due 2025 [Member]
Description of accounting policies [Line Items]
Debt Instrument, Interest Rate, Stated Percentage						8.00%