Debt (Details) (USD $)	3 Months Ended
	Mar. 31, 2015		Dec. 31, 2014
Debt Instrument [Line Items]
Total debt	$ 1,296,400,000		$ 1,296,400,000
Less current portion of debt	(61,900,000)		(61,900,000)
Total long-term debt	1,234,500,000		1,234,500,000
Holding company cash and investments	494,000,000
Unrealized gain loss on holding company investments	1,300,000
Modified duration of holding company investments	2 years 9 months 18 days
Period preceding election to convert	5 days
5.375% Senior Notes [Member]
Debt Instrument [Line Items]
Total debt	61,900,000		61,900,000
5.375% Senior Notes [Member] | Senior Notes Due 2015 [Member]
Debt Instrument [Line Items]
Stated interest rate (in hundredths)	5.375%
Maturity date	Nov. 01, 2015
5% Convertible Senior Notes due 2017 [Member]
Debt Instrument [Line Items]
Total debt	345,000,000	[1]	345,000,000	[1]
Stated interest rate (in hundredths)	5.00%
Maturity date	May 01, 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
2% Convertible Senior Notes due 2020 [Member]
Debt Instrument [Line Items]
Total debt	500,000,000	[2],[3]	500,000,000	[2],[3]
Stated interest rate (in hundredths)	2.00%
Maturity date	Apr. 01, 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
Minimum number of trading days	20 days
Maximum number of trading days	30 days
Percentage of conversion price (in hundredths)	130.00%
130% of conversion price (in dollars per share)	$ 9.03
9% Convertible Junior Subordinated Debentures due 2063 [Member]
Debt Instrument [Line Items]
Total debt	389,500,000	[4]	389,500,000	[4]
Maturity date	Apr. 01, 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

[1] Convertible at any time prior to maturity at the holder's option, at an initial conversion rate, which is subject to adjustment, of 74.4186 shares per $1,000 principal amount, representing an initial conversion price of approximately $13.44 per share.

[2] Prior to January 1, 2020, the 2% Convertible Senior Notes are convertible only upon satisfaction of one or more conditions. One such condition is that during any calendar quarter commencing after March 31, 2014, the last reported sale price of our common stock for each of at least 20 trading days during the 30 consecutive trading days ending on, and including, the last trading day of the immediately preceding calendar quarter be greater than or equal to 130% of the applicable conversion price on each applicable trading day. The 2% Notes are convertible at an initial conversion rate, which is subject to adjustment, of 143.8332 shares per $1,000 principal amount, representing an initial conversion price of approximately $6.95 per share. 130% of such conversion price is $9.03. On or after January 1, 2020, holders may convert their notes irrespective of satisfaction of the conditions. For the quarter ending March 31, 2015, our common stock was greater than or equal to 130% of the applicable conversion price for each of at least 20 trading days during the 30 consecutive trading days ending on, and including, March 31, 2015.

[3] Prior to April 10, 2017, the notes will not be redeemable. On any business day on or after April 10, 2017 we may redeem for cash all or part of the notes, at our option, at a redemption rate equal to 100% of the principal amount of the notes being redeemed, plus any accrued and unpaid interest, if the closing sale price of our common stock exceeds 130% of the then prevailing conversion price of the notes for each of at least 20 of the 30 consecutive trading days preceding notice of the redemption.

[4] Convertible at any time prior to maturity at the holder's option, at an initial conversion rate, which is subject to adjustment, of 74.0741 share per $1,000 principal amount, representing an initial conversion price of approximately $13.50 per share. If a holder elects to convert their debentures, deferred interest owed on the debentures being converted is also converted into shares of our common stock. The conversion rate for any deferred interest is based on the average price that our shares traded at during a 5-day period immediately prior to the election to convert. In lieu of issuing shares of common stock upon conversion of the debentures, we may, at our option, make a cash payment to converting holders for all or some of the shares of our common stock otherwise issuable upon conversion.