Debt (Details) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Debt Instrument [Line Items]
Net proceeds from convertible senior notes	$ 0	$ 484,670,000
Holding company cash and investments	542,000,000
Unrealized gain loss on holding company investments	7,100,000
Modified duration of holding company investments	3 years 4 months 24 days
Par Value [Member]
Liabilities [Abstract]
Senior Notes	61,953,000		82,883,000
Convertible Senior Notes due 2017	345,000,000		345,000,000
Convertible Senior Notes due 2020	500,000,000		500,000,000
Convertible Junior Subordinated Debentures	389,522,000		389,522,000
Total Debt	1,296,475,000		1,317,405,000
Fair Value [Member]
Liabilities [Abstract]
Senior Notes	64,121,000		85,991,000
Convertible Senior Notes due 2017	397,599,000		388,988,000
Convertible Senior Notes due 2020	703,815,000		685,625,000
Convertible Junior Subordinated Debentures	464,018,000		439,186,000
Total Debt	1,629,553,000		1,599,790,000
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Liabilities [Abstract]
Senior Notes	64,121,000		85,991,000
Convertible Senior Notes due 2017	397,599,000		388,988,000
Convertible Senior Notes due 2020	703,815,000		685,625,000
Convertible Junior Subordinated Debentures	0		0
Total Debt	1,165,535,000		1,160,604,000
Significant Other Observable Inputs (Level 2) [Member]
Liabilities [Abstract]
Senior Notes	0		0
Convertible Senior Notes due 2017	0		0
Convertible Senior Notes due 2020	0		0
Convertible Junior Subordinated Debentures	464,018,000		439,186,000
Total Debt	464,018,000		439,186,000
Significant Unobservable Inputs (Level 3) [Member]
Liabilities [Abstract]
Senior Notes	0		0
Convertible Senior Notes due 2017	0		0
Convertible Senior Notes due 2020	0		0
Convertible Junior Subordinated Debentures	0		0
Total Debt	0		0
Senior Notes [Member]
Debt Instrument [Line Items]
Repurchase of debt instrument	20,900,000
Interest payments made	300,000
Senior Notes [Member] | Senior Notes Due 2015 [Member]
Debt Instrument [Line Items]
Outstanding principal amount	62,000,000		82,900,000
Stated interest rate (in hundredths)	5.375%		5.375%
Maturity date	November 1, 2015		November 1, 2015
Convertible Senior Notes Due 2017 [Member]
Debt Instrument [Line Items]
Outstanding principal amount	345,000,000		345,000,000
Stated interest rate (in hundredths)	5.00%		5.00%
Maturity date	May 1, 2017		May 1, 2017
Conversion rate (in shares per $1,000 note)	74.4186
Principal amount of notes used in determining conversion rate	1,000
Initial conversion price (in dollars per share)	$ 13.44
Convertible Senior Notes - due April 2020 [Member]
Debt Instrument [Line Items]
Outstanding principal amount	500,000,000		500,000,000
Stated interest rate (in hundredths)	2.00%		2.00%
Maturity date	April 1, 2020		April 1, 2020
Conversion rate (in shares per $1,000 note)	143.8332
Principal amount of notes used in determining conversion rate	1,000
Initial conversion price (in dollars per share)	$ 6.95
130% of conversion price (in dollars per share)	$ 9.03
Net proceeds from convertible senior notes		484,600,000
Redemption price, percentage (in hundredths)	100.00%
Percentage of conversion price (in hundredths)	130.00%
Minimum number of trading days	20 days
Maximum number of trading days	30 days
Convertible Junior Subordinated Debentures Due 2063 [Member]
Debt Instrument [Line Items]
Outstanding principal amount	389,500,000		389,500,000
Stated interest rate (in hundredths)	9.00%		9.00%
Maturity date	April 1, 2063		April 1, 2063
Conversion rate (in shares per $1,000 note)	74.0741
Principal amount of notes used in determining conversion rate	1,000
Initial conversion price (in dollars per share)	$ 13.5
Redemption price, percentage (in hundredths)	100.00%
Percentage of conversion price (in hundredths)	130.00%
Minimum number of trading days	20 days
Maximum number of trading days	30 days
Minimum number of consecutive interest periods for which interest payments may be deferred	1
Maximum period for which interest payments may be deferred without giving rise to an event of default	10 years
Interest payment deferred		18,300,000
Interest payment due on the debentures		$ 17,500,000
Period preceding election to convert	5 days