Fair Value Measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
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	$ 103,700	$ 73,530
Obligations of U.S. states and political subdivisions	2,113,283	2,143,016
Corporate debt securities	1,875,255	1,743,910
ABS	9,860	59,565
RMBS	192,896	224,209
CMBS	305,425	319,817
CLOs	109,734	121,175
Total debt securities	4,710,153	4,685,222
Equity securities	7,239	7,128
Total investment portfolio	4,717,392	4,692,350
Real estate acquired	11,728	11,748
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	13,312	30,690
Obligations of U.S. states and political subdivisions	0	0
Corporate debt securities	0	0
ABS	0	0
RMBS	0	0
CMBS	0	0
CLOs	0	0
Total debt securities	13,312	30,690
Equity securities	2,971	2,860
Total investment portfolio	16,283	33,550
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	90,388	42,840
Obligations of U.S. states and political subdivisions	2,112,903	2,142,325
Corporate debt securities	1,875,255	1,743,910
ABS	9,860	59,565
RMBS	192,896	224,209
CMBS	305,425	319,817
CLOs	109,734	121,175
Total debt securities	4,696,461	4,653,841
Equity securities	0	0
Total investment portfolio	4,696,461	4,653,841
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	380	691
Corporate debt securities	0	0
ABS	0	0
RMBS	0	0
CMBS	0	0
CLOs	0	0
Total debt securities	380	691
Equity securities	4,268	4,268
Total investment portfolio	4,648	4,959
Real estate acquired	$ 11,728	$ 11,748