Fair Value Measurements (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Annual average discount rate (in hundredths)	7.10%
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	$ 61,330	$ 160,584
Obligations of U.S. states and political subdivisions	2,077,452	1,792,527
Corporate debt securities	1,729,965	2,004,763
Asset-backed securities	104,407	116,617
Residential mortgage-backed securities	238,340	257,648
Commercial mortgage-backed securities	284,158	233,491
Collateralized loan obligations	60,550	60,200
Debt securities issued by foreign sovereign governments		31,731
Total debt securities	4,556,202	4,657,561
Equity securities	9,876	5,645
Total investment portfolio	4,566,078	4,663,206
Real estate acquired	9,642	12,149
Carrying Value
Debt [Abstract]
FHLB Advance due 2023	155,000
Convertible Senior Notes due 2017	144,470	331,546
Convertible Senior Notes due 2020	491,854	490,755
Convertible Junior Subordinated Debentures due 2063	256,872	389,522
Total Debt	1,048,196	1,211,823
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	14,678	46,197
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		31,731
Total debt securities	14,678	77,928
Equity securities	2,936	2,790
Total investment portfolio	17,614	80,718
Real estate acquired	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	46,652	114,387
Obligations of U.S. states and political subdivisions	2,076,396	1,791,299
Corporate debt securities	1,729,965	2,004,763
Asset-backed securities	104,407	116,617
Residential mortgage-backed securities	238,340	257,648
Commercial mortgage-backed securities	284,158	233,491
Collateralized loan obligations	60,550	60,200
Debt securities issued by foreign sovereign governments		0
Total debt securities	4,540,468	4,578,405
Equity securities	0	0
Total investment portfolio	4,540,468	4,578,405
Real estate acquired	0	0
Debt [Abstract]
FHLB Advance due 2023	160,025
Convertible Senior Notes due 2017	150,070	345,616
Convertible Senior Notes due 2020	554,670	701,955
Convertible Junior Subordinated Debentures due 2063	285,244	455,067
Total Debt	1,150,009	1,502,638
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	1,056	1,228
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
Total debt securities	1,056	1,228
Equity securities	6,940	2,855
Total investment portfolio	7,996	4,083
Real estate acquired	$ 9,642	$ 12,149