Debt and Other Financing (Narrative) (Details) - USD ($)		1 Months Ended						3 Months Ended							12 Months Ended
	Jan. 03, 2020	Nov. 30, 2020	Sep. 30, 2020	Dec. 31, 2019	Jul. 31, 2019	Jun. 30, 2018	Aug. 31, 2016	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2016	Mar. 31, 2015	Mar. 31, 2013	Mar. 31, 2008
Debt Instrument [Line Items]
Gain (loss) on extinguishment of debt								$ (25,600,000)							$ (37,700,000)	$ (11,600,000)	$ (700,000)
Write off of debt issuance cost								7,900,000
Loss on debt extinguishment								$ 17,700,000
Fourth Quarter 2018 Repurchase
Debt Instrument [Line Items]
Payments for debt extinguishment accrued interest													$ 700,000
Gain (loss) on extinguishment of debt													2,100,000
Write off of debt issuance cost													300,000
Debt repurchase discount received													2,400,000				2,400,000
2013 Notes
Debt Instrument [Line Items]
Increase in interest rate for every one-notch downgrade of long-term credit ratings below investment grade															0.25%
Maximum aggregate increase in interest rate related to downgrade of long-term credit ratings below investment grade															2.00%
2015 Facility
Debt Instrument [Line Items]
Write off of debt issuance cost	$ 7,800,000											$ 2,000,000.0
4.60% Notes, due March 2020
Debt Instrument [Line Items]
Principal amount of debt					$ 274,800,000															$ 500,000,000.0
Interest rate, stated percentage				4.60%	4.60%					4.60%						4.60%		4.60%	4.60%	4.60%
Repayments of debt							$ 50,100,000
Repurchase amount					$ 274,800,000								23,000,000.0				23,000,000.0	$ 40,000,000.0
Repayments of long-term debt				$ 112,200,000
Payments for debt extinguishment accrued interest				1,700,000
Gain (loss) on extinguishment of debt										$ (1,500,000)	$ (8,100,000)
Debt repurchase discount received					600,000
Debt repurchase, premium paid					8,200,000
Accrued interest paid on extinguishment of debt					$ 5,400,000
Make whole premium				1,400,000						1,400,000						$ 1,400,000
5.00% Notes, due March 2023
Debt Instrument [Line Items]
Principal amount of debt																				$ 500,000,000.0
Interest rate, stated percentage					5.00%			5.00%							5.00%			5.00%	5.00%	5.00%
Repurchase amount													$ 27,000,000.0				$ 27,000,000.0	$ 11,100,000
6.95% Notes, due March 2043
Debt Instrument [Line Items]
Principal amount of debt																				$ 250,000,000.0
Interest rate, stated percentage			6.95%					6.95%	6.95%		6.95%				6.95%			6.95%		6.95%
Repurchase amount			$ 27,800,000						$ 27,800,000									$ 6,200,000
Gain (loss) on extinguishment of debt									(4,100,000)
Write off of debt issuance cost			300,000
Make whole premium			3,800,000						$ 3,800,000
Accrued interest			$ 1,200,000
6.50% Notes, due March 2019
Debt Instrument [Line Items]
Principal amount of debt																					$ 350,000,000.0
Interest rate, stated percentage					6.50%	6.50%								6.50%				6.50%			6.50%
Repayments of debt							68,100,000
Repurchase amount																		$ 44,000,000.0
Repayments of long-term debt						$ 237,800,000
Payment for debt extinguishment or debt prepayment cost						6,200,000								$ 6,200,000
Payments for debt extinguishment accrued interest						$ 4,600,000
Gain (loss) on extinguishment of debt														(2,900,000)
Write off of debt issuance cost														300,000
Unsecured Notes
Debt Instrument [Line Items]
Accrued interest				15,900,000				$ 15,200,000		15,900,000					$ 15,200,000	15,900,000
7.875% Senior Secured Notes, due August 2022 (2016 Notes)
Debt Instrument [Line Items]
Principal amount of debt							$ 500,000,000.0
Interest rate, stated percentage							7.875%	7.875%							7.875%
Repayment of principal		$ 500,000,000
Payment of premium		9,800,000
Payment of accrued interest		8,400,000
6.50% Senior Secured Notes, due August 2022
Debt Instrument [Line Items]
Principal amount of debt					$ 400,000,000.0
Interest rate, stated percentage								6.50%							6.50%
Repayment of principal		400,000,000
Payment of premium		7,900,000
Payment of accrued interest		$ 5,600,000
Senior Notes
Debt Instrument [Line Items]
Accrued interest				$ 24,700,000						$ 24,700,000						$ 24,700,000
Interest Rate Swap | 6.50% Notes, due March 2019
Debt Instrument [Line Items]
Deferred gain on termination of contract														$ 3,600,000