Fair Value Measurements (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
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	$ 189,062	$ 178,846
Obligations of U.S. states and political subdivisions	2,102,697	2,152,524
Corporate debt securities	2,056,079	2,066,838
ABS	9,422	4,923
RMBS	171,540	181,849
CMBS	293,356	297,312
CLOs	107,907	101,023
Total fixed income securities	4,930,063	4,983,315
Equity securities	4,099	7,246
Total investments at fair value	4,934,162	4,990,561
Real estate acquired	10,078	12,713
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	81,418	81,598
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 fixed income securities	81,418	81,598
Equity securities	2,931	2,978
Total investments at fair value	84,349	84,576
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	107,644	97,248
Obligations of U.S. states and political subdivisions	2,102,443	2,152,253
Corporate debt securities	2,056,079	2,066,838
ABS	9,422	4,923
RMBS	171,540	181,849
CMBS	293,356	297,312
CLOs	107,907	101,023
Total fixed income securities	4,848,391	4,901,446
Equity securities	0	0
Total investments at fair value	4,848,391	4,901,446
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	254	271
Corporate debt securities	0	0
ABS	0	0
RMBS	0	0
CMBS	0	0
CLOs	0	0
Total fixed income securities	254	271
Equity securities	1,168	4,268
Total investments at fair value	1,422	4,539
Real estate acquired	$ 10,078	$ 12,713