Debt (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	6 Months Ended													6 Months Ended				6 Months Ended			6 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2013 Par Value [Member]	Dec. 31, 2012 Par Value [Member]	Jun. 30, 2013 Fair Value [Member]	Dec. 31, 2012 Fair Value [Member]	Jun. 30, 2013 Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Dec. 31, 2012 Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Jun. 30, 2013 Significant Other Observable Inputs (Level 2) [Member]	Dec. 31, 2012 Significant Other Observable Inputs (Level 2) [Member]	Jun. 30, 2013 Significant Unobservable Inputs (Level 3) [Member]	Dec. 31, 2012 Significant Unobservable Inputs (Level 3) [Member]	Jun. 30, 2013 Senior Notes [Member]	Jun. 30, 2012 Senior Notes [Member]	Jun. 30, 2013 Senior Notes [Member] Senior Notes Due 2015 [Member]	Dec. 31, 2012 Senior Notes [Member] Senior Notes Due 2015 [Member]	Jun. 30, 2013 Convertible Senior Notes Due 2017 [Member]	Jun. 30, 2012 Convertible Senior Notes Due 2017 [Member]	Dec. 31, 2012 Convertible Senior Notes Due 2017 [Member]	Jun. 30, 2013 Convertible Senior Notes - due April 2020 [Member]	Mar. 31, 2013 Convertible Senior Notes - due April 2020 [Member]	Jun. 30, 2013 Convertible Junior Subordinated Debentures Due 2063 [Member] Period	Jun. 30, 2012 Convertible Junior Subordinated Debentures Due 2063 [Member]	Dec. 31, 2012 Convertible Junior Subordinated Debentures Due 2063 [Member]
Debt Instrument [Line Items]
Outstanding principal amount																$ 82,900,000	$ 100,100,000	$ 345,000,000		$ 345,000,000	$ 500,000,000	$ 500,000,000	$ 389,500,000		$ 389,500,000
Repurchase of debt instrument														17,200,000
Stated interest rate (in hundredths)																5.375%	5.375%	5.00%		5.00%	2.00%	2.00%	9.00%		9.00%
Maturity date																November, 2015	November, 2015	May 1, 2017			April 1, 2020		April 1, 2063
Minimum percentage of consolidated shareholders' equity that determines designated subsidiary status (in hundredths)														15.00%
Restriction on acceleration of other debt														40,000,000				40,000,000			40,000,000
Default period														30 days				30 days			30 days
Minimum percentage of aggregate notes held to grant acceleration rights to note holders (in hundredths)														25.00%				25.00%			25.00%		25.00%
Interest payments made														2,800,000	4,800,000			8,600,000	8,600,000				35,800,000	17,500,000
Debt instrument description of event of default	we fail to meet any of the covenants of the 5% Notes and such failure continues for 60 days after we receive notice from holders of 25% or more of the 5% Notes
Restriction on final judgments																		40,000,000			40,000,000
Conversion rate (in shares per $1,000 note)																		74.4186			143.8332		74.0741
Principal amount of notes used in determining conversion rate																		1,000			1,000		1,000
Initial conversion price (in dollars per share)																		$ 13.44			$ 6.95		$ 13.50
Net proceeds from convertible senior notes	484,697,000	0																			484,697,000
Redemption price, percentage (in hundredths)																					100.00%		100.00%
Percentage of conversion price (in hundredths)																					130.00%		130.00%
Minimum number of trading days																					20 days		20 days
Maximum number of trading days																					30 days		30 days
Amortized value of notes recorded as a liability on the consolidated balance sheet	389,522,000		379,609,000																						379,600,000
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
Annual rate (in hundredths)																5.375%	5.375%	5.00%		5.00%	2.00%	2.00%	9.00%		9.00%
Interest payment deferred																							18,300,000
Interest payment due on the debentures																							17,500,000
Period in which reasonable commercial efforts must begin, maximum																							1 day
Anniversary payment release of the start of the interest deferral to the Alternative Payment Mechanism in lieu of the final maturity of the debentures																							tenth
Period in which reasonable commercial efforts must begin, minimum																							fifth anniversary of the interest payment date
Net proceeds cap (in hundredths)																							2.00%
Maximum number of shares of common stock issuable under the Alternative Payment Mechanism (in shares)																							10
Maximum percentage of aggregate principal amount of the debentures (in hundredths)																							25.00%
Period preceding election to convert																							5 days
Holding company cash and investments	592,000,000
Unrealized gain loss on holding company investments	7,100,000
Modified duration of holding company investments	3 years 7 months 6 days
Liabilities [Abstract]
Senior Notes				82,883,000	100,118,000	83,753,000	79,594,000	83,753,000	79,594,000	0	0	0	0
Convertible Senior Notes due 2017				345,000,000	345,000,000	354,919,000	242,880,000	354,919,000	242,880,000	0	0	0	0
Convertible Senior Notes due 2020				500,000,000		580,000,000		580,000,000		0		0
Convertible Junior Subordinated Debentures				389,522,000	389,522,000	426,285,000	173,096,000	0	0	426,285,000	173,096,000	0	0
Total Debt				$ 1,317,405,000	$ 834,640,000	$ 1,444,957,000	$ 495,570,000	$ 1,018,672,000	$ 322,474,000	$ 426,285,000	$ 173,096,000	$ 0	$ 0