Fair Value of Financial Instruments (Details) - USD ($)	Mar. 31, 2017	Dec. 31, 2016
Financial Assets:
Total investments	$ 1,724,533,000	$ 1,615,102,000
U.S. Treasury securities
Financial Assets:
Total investments	178,743,000	191,548,000
U.S. agency securities
Financial Assets:
Total investments	26,284,000	18,441,000
Municipal debt securities
Financial Assets:
Total investments	361,392,000	334,324,000
Corporate debt securities
Financial Assets:
Total investments	505,288,000	456,357,000
Residential and commercial mortgage securities
Financial Assets:
Total investments	65,288,000	68,336,000
Asset-backed securities
Financial Assets:
Total investments	134,911,000	127,172,000
Money market funds
Financial Assets:
Total investments	97,388,000	102,430,000
U.S. Agency | U.S. agency mortgage-backed securities
Financial Assets:
Total investments	355,239,000	316,494,000
Recurring basis
Financial Assets:
Assets fair value	1,724,533,000	1,615,102,000
Recurring basis | U.S. Treasury securities
Financial Assets:
Total investments	178,743,000	191,548,000
Recurring basis | U.S. agency securities
Financial Assets:
Total investments	26,284,000	18,441,000
Recurring basis | Municipal debt securities
Financial Assets:
Total investments	361,392,000	334,324,000
Recurring basis | Corporate debt securities
Financial Assets:
Total investments	505,288,000	456,357,000
Recurring basis | Residential and commercial mortgage securities
Financial Assets:
Total investments	65,288,000	68,336,000
Recurring basis | Asset-backed securities
Financial Assets:
Total investments	134,911,000	127,172,000
Recurring basis | Money market funds
Financial Assets:
Total investments	97,388,000	102,430,000
Recurring basis | U.S. Agency | U.S. agency mortgage-backed securities
Financial Assets:
Total investments	355,239,000	316,494,000
Quoted Prices in Active Markets for Identical Instruments (Level 1) | Recurring basis
Financial Assets:
Assets fair value	276,131,000	293,978,000
Quoted Prices in Active Markets for Identical Instruments (Level 1) | Recurring basis | U.S. Treasury securities
Financial Assets:
Total investments	178,743,000	191,548,000
Quoted Prices in Active Markets for Identical Instruments (Level 1) | Recurring basis | Money market funds
Financial Assets:
Total investments	97,388,000	102,430,000
Significant Other Observable Inputs (Level 2) | Recurring basis
Financial Assets:
Assets fair value	1,448,402,000	1,321,124,000
Significant Other Observable Inputs (Level 2) | Recurring basis | U.S. agency securities
Financial Assets:
Total investments	26,284,000	18,441,000
Significant Other Observable Inputs (Level 2) | Recurring basis | Municipal debt securities
Financial Assets:
Total investments	361,392,000	334,324,000
Significant Other Observable Inputs (Level 2) | Recurring basis | Corporate debt securities
Financial Assets:
Total investments	505,288,000	456,357,000
Significant Other Observable Inputs (Level 2) | Recurring basis | Residential and commercial mortgage securities
Financial Assets:
Total investments	65,288,000	68,336,000
Significant Other Observable Inputs (Level 2) | Recurring basis | Asset-backed securities
Financial Assets:
Total investments	134,911,000	127,172,000
Significant Other Observable Inputs (Level 2) | Recurring basis | U.S. Agency | U.S. agency mortgage-backed securities
Financial Assets:
Total investments	355,239,000	316,494,000
Significant Unobservable Inputs (Level 3) | Recurring basis
Financial Assets:
Assets fair value	$ 0	$ 0