Fair Value Measurements (Details) - USD ($) $ in Thousands		Sep. 30, 2015		Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. Treasury securities and obligations of U.S. government corporations and agencies		$ 104,966		$ 346,775
Obligations of U.S. states and political subdivisions		1,612,521		855,142
Corporate debt securities		2,120,572		2,425,281
Asset-backed securities		132,537		286,655
Residential mortgage-backed securities		271,471		321,237
Commercial mortgage-backed securities		252,687		275,278
Collateralized loan obligations		60,286		60,076
Debt securities issued by foreign sovereign governments		33,291		39,170
Total debt securities		4,588,331		4,609,614
Equity securities	[1]	5,634		3,055
Total investment portfolio		4,593,965		4,612,669
Real estate acquired		10,084	[2]	12,658	[1]
Par Value
Debt [Abstract]
Senior Notes		61,953		61,953
Convertible Senior Notes due 2017		345,000		345,000
Convertible Senior Notes due 2020		500,000		500,000
Convertible Junior Subordinated Debentures		389,522		389,522
Total Debt		1,296,475		1,296,475
Total Fair Value
Debt [Abstract]
Senior Notes		62,027		63,618
Convertible Senior Notes due 2017		366,749		387,997
Convertible Senior Notes due 2020		719,375		735,075
Convertible Junior Subordinated Debentures		485,648		500,201
Total Debt		1,633,799		1,686,891
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. Treasury securities and obligations of U.S. government corporations and agencies		18,418		188,824
Obligations of U.S. states and political subdivisions		0		0
Corporate debt securities		0		0
Asset-backed securities		0		0
Residential mortgage-backed securities		0		0
Commercial mortgage-backed securities		0		0
Collateralized loan obligations		0		0
Debt securities issued by foreign sovereign governments		33,291		39,170
Total debt securities		51,709		227,994
Equity securities	[1]	2,778		2,734
Total investment portfolio		54,487		230,728
Real estate acquired		0	[2]	0	[1]
Debt [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
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. Treasury securities and obligations of U.S. government corporations and agencies		86,548		157,951
Obligations of U.S. states and political subdivisions		1,611,134		853,296
Corporate debt securities		2,120,572		2,425,281
Asset-backed securities		132,537		286,655
Residential mortgage-backed securities		271,471		321,237
Commercial mortgage-backed securities		252,687		275,278
Collateralized loan obligations		60,286		60,076
Debt securities issued by foreign sovereign governments		0		0
Total debt securities		4,535,235		4,379,774
Equity securities	[1]	0		0
Total investment portfolio		4,535,235		4,379,774
Real estate acquired		0	[2]	0	[1]
Debt [Abstract]
Senior Notes		62,027		63,618
Convertible Senior Notes due 2017		366,749		387,997
Convertible Senior Notes due 2020		719,375		735,075
Convertible Junior Subordinated Debentures		485,648		500,201
Total Debt		1,633,799		1,686,891
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. Treasury securities and obligations of U.S. government corporations and agencies		0		0
Obligations of U.S. states and political subdivisions		1,387		1,846
Corporate debt securities		0		0
Asset-backed securities		0		0
Residential mortgage-backed securities		0		0
Commercial mortgage-backed securities		0		0
Collateralized loan obligations		0		0
Debt securities issued by foreign sovereign governments		0		0
Total debt securities		1,387		1,846
Equity securities	[1]	2,856		321
Total investment portfolio		4,243		2,167
Real estate acquired		10,084	[2]	12,658	[1]
Debt [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

[1] Equity securities in Level 3 are carried at cost, which approximates fair value.

[2] Real estate acquired through claim settlement, which is held for sale, is reported in Other assets on the consolidated balance sheets.