Fair Value Measurements (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2016	Dec. 31, 2015
Fair Value Disclosures [Abstract]
State premium tax credit investments, average maturity		2 years
Annual average discount rate (in hundredths)		7.20%
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		$ 148,528	$ 160,584
Obligations of U.S. states and political subdivisions		1,913,130	1,792,527
Corporate debt securities		1,854,207	2,004,763
Asset-backed securities		110,328	116,617
Residential mortgage-backed securities		250,641	257,648
Commercial mortgage-backed securities		220,696	233,491
Collateralized loan obligations		60,384	60,200
Debt securities issued by foreign sovereign governments			31,731
Total debt securities		4,557,914	4,657,561
Equity securities	[1]	6,289	5,645
Total investment portfolio		4,564,203	4,663,206
Real estate acquired	[2]	12,849	12,149
Carrying Value
Debt [Abstract]
FHLB Advance due 2023		155,000
Convertible Senior Notes due 2017		194,319	331,546
Convertible Senior Notes due 2020		491,305	490,755
Convertible Junior Subordinated Debentures due 2063		256,872	389,522
Total Debt		1,097,496	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		51,448	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		51,448	77,928
Equity securities	[1]	2,868	2,790
Total investment portfolio		54,316	80,718
Real estate acquired	[2]	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		97,080	114,387
Obligations of U.S. states and political subdivisions		1,911,938	1,791,299
Corporate debt securities		1,854,207	2,004,763
Asset-backed securities		110,328	116,617
Residential mortgage-backed securities		250,641	257,648
Commercial mortgage-backed securities		220,696	233,491
Collateralized loan obligations		60,384	60,200
Debt securities issued by foreign sovereign governments			0
Total debt securities		4,505,274	4,578,405
Equity securities	[1]	0	0
Total investment portfolio		4,505,274	4,578,405
Real estate acquired	[2]	0	0
Debt [Abstract]
FHLB Advance due 2023		156,831
Convertible Senior Notes due 2017		203,771	345,616
Convertible Senior Notes due 2020		630,310	701,955
Convertible Junior Subordinated Debentures due 2063		292,754	455,067
Total Debt		1,283,666	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		1,192	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		1,192	1,228
Equity securities	[1]	3,421	2,855
Total investment portfolio		4,613	4,083
Real estate acquired	[2]	$ 12,849	$ 12,149

[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.