Fair Value Measurements (Details) - USD ($) $ in Thousands		Jun. 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		$ 129,556	$ 346,775
Obligations of U.S. states and political subdivisions		1,306,860	855,142
Corporate debt securities		2,266,946	2,425,281
Asset-backed securities		200,267	286,655
Residential mortgage-backed securities		287,021	321,237
Commercial mortgage-backed securities		261,668	275,278
Collateralized loan obligations		60,683	60,076
Debt securities issued by foreign sovereign governments		36,046	39,170
Total debt securities		4,549,047	4,609,614
Equity securities		3,063	3,055
Total investment portfolio		4,552,110	4,612,669
Real estate acquired	[1]	7,995	12,658
Par Value [Member]
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
Fair Value [Member]
Debt [Abstract]
Senior Notes		62,689	63,618
Convertible Senior Notes due 2017		394,521	387,997
Convertible Senior Notes due 2020		837,125	735,075
Convertible Junior Subordinated Debentures		512,186	500,201
Total Debt		1,806,521	1,686,891
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
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		26,271	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		36,046	39,170
Total debt securities		62,317	227,994
Equity securities		2,742	2,734
Total investment portfolio		65,059	230,728
Real estate acquired	[1]	0	0
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) [Member]
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		103,285	157,951
Obligations of U.S. states and political subdivisions		1,305,226	853,296
Corporate debt securities		2,266,946	2,425,281
Asset-backed securities		200,267	286,655
Residential mortgage-backed securities		287,021	321,237
Commercial mortgage-backed securities		261,668	275,278
Collateralized loan obligations		60,683	60,076
Debt securities issued by foreign sovereign governments		0	0
Total debt securities		4,485,096	4,379,774
Equity securities		0	0
Total investment portfolio		4,485,096	4,379,774
Real estate acquired	[1]	0	0
Debt [Abstract]
Senior Notes		62,689	63,618
Convertible Senior Notes due 2017		394,521	387,997
Convertible Senior Notes due 2020		837,125	735,075
Convertible Junior Subordinated Debentures		512,186	500,201
Total Debt		1,806,521	1,686,891
Significant Unobservable Inputs (Level 3) [Member]
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		1,634	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		1,634	1,846
Equity securities		321	321
Total investment portfolio		1,955	2,167
Real estate acquired	[1]	7,995	12,658
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] Real estate acquired through claim settlement, which is held for sale, is reported in Other assets on the consolidated balance sheets.