Description of Business and Accounting Policies Description of Business and Accounting Policies (Details) $ / shares in Units, shares in Millions	3 Months Ended				12 Months Ended
	Dec. 31, 2015 USD ($) shares	Sep. 30, 2015 USD ($) $ / shares shares	Jun. 30, 2015 USD ($) $ / shares shares	Jun. 30, 2014 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) d shares	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Jan. 24, 2013 shares
Description of accounting policies [Line Items]
Carrying Value of Investment in CyrusOne	$ 55,500,000				$ 55,500,000	$ 273,600,000
Threshold for Cost Method Investments, Percentage	20.00%				20.00%
Entity Number of Employees	3,250				3,250
Equity Method Investment Sold, common stock | shares	1.4
Sale of Equity Method Invesment Common Stock, Price Per Share	$ 0
Proceeds from Sale of Equity Method Investments	$ 47,600,000	$ 170,300,000	$ 426,000,000	$ 355,900,000	$ 643,900,000	$ 355,900,000	$ 0
Percent of Revenue from Foreign Subsidiaries					1.00%
Number of Operating Segments					2	2	2
Investment owned in CyrusOne Inc., Balance, Shares | shares								1.9
Investment owned in CyrusOne Inc., Percentage, Common Stock								8.60%
Investment owned in Equity Method Investee, Balance, Partnership Units | shares	6.3				6.3			42.6
Cost Method Investment, Ownership Percentage	9.50%				9.50%
Investment owned in CyrusOne LP, Percentage, Partnership Units								66.00%
Total ownership interests in CyrusOne, Percentage								69.00%
Cost Method Investments	$ 55,500,000				$ 55,500,000
Equity Method Investment Sold, Partnership Units | shares		6.0	14.3	16.0
Sale of Partnership Units, Price Per Unit | $ / shares		$ 28.41	$ 29.88	$ 22.26
Proceeds from sale of wireless spectrum licenses					$ 0	$ 194,400,000	$ 0
Trade Receivables Due From Customers, Range, Minimum | d					21
Trade Receivables Due from Customers, Range, Maximum | d					90
Number of customers, exceeds 10% of total accounts receivable					1
Accounts Receivable from one customer greater than 10%, percentage	22.00%				22.00%	26.00%
Revenue from one customer greater than 10%, percentage	12.00%				12.00%	14.00%
Unbilled Receivables, Current	$ 14,000,000				$ 14,000,000	$ 13,200,000
Dividends received from CyrusOne					22,200,000	28,400,000	21,300,000
Payments to Acquire Equity Method Investments					$ 5,500,000
Customer Contract, Lower Range, in Years					1
Customer Contract, Upper Range, in Years					3
Advertising Expense					$ 8,300,000	7,200,000	7,300,000
Regulatory Taxes Included in Revenue					15,500,000	15,200,000	14,900,000
Regulatory Taxes Included in Expense					$ 17,900,000	$ 16,400,000	$ 14,900,000
Lower range, in years, of remaining service life of active employees	9				9
Upper range, in years, of remaining service life of active employees	13				13
Company Employees Participating in Collective Bargaining Agreement, Percentage	30.00%				30.00%
Other Postretirement Benefit Plans, Defined Benefit [Member]
Description of accounting policies [Line Items]
Average life expectancy of retirees, in years	17				17