Fair Value Measurements (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Fair Value Disclosures [Abstract]
State premium tax credit investments, average maturity	3 years
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	$ 42,256	$ 160,584
Obligations of U.S. states and political subdivisions	2,132,669	1,792,527
Corporate debt securities	1,749,869	2,004,763
Asset-backed securities	79,182	116,617
Residential mortgage-backed securities	243,436	257,648
Commercial mortgage-backed securities	350,213	233,491
Collateralized loan obligations	121,000	60,200
Debt securities issued by foreign sovereign governments		31,731
Total debt securities	4,718,625	4,657,561
Equity securities	7,218	5,645
Total investment portfolio	4,725,843	4,663,206
Real estate acquired	10,564	12,149
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	8,755	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	8,755	77,928
Equity securities	2,950	2,790
Total investment portfolio	11,705	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	33,501	114,387
Obligations of U.S. states and political subdivisions	2,131,839	1,791,299
Corporate debt securities	1,749,869	2,004,763
Asset-backed securities	79,182	116,617
Residential mortgage-backed securities	243,436	257,648
Commercial mortgage-backed securities	350,213	233,491
Collateralized loan obligations	121,000	60,200
Debt securities issued by foreign sovereign governments		0
Total debt securities	4,709,040	4,578,405
Equity securities	0	0
Total investment portfolio	4,709,040	4,578,405
Real estate acquired	0	0
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	830	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	830	1,228
Equity securities	4,268	2,855
Total investment portfolio	5,098	4,083
Real estate acquired	$ 10,564	$ 12,149