Fair Value of Financial Instruments (Details) - USD ($)	Jun. 30, 2017	Dec. 31, 2016
Financial Assets:
Total investments	$ 1,841,041,000	$ 1,615,102,000
U.S. Treasury securities
Financial Assets:
Total investments	202,364,000	191,548,000
Municipal debt securities
Financial Assets:
Total investments	370,068,000	334,324,000
Corporate debt securities
Financial Assets:
Total investments	555,965,000	456,357,000
Residential and commercial mortgage securities
Financial Assets:
Total investments	69,672,000	68,336,000
Asset-backed securities
Financial Assets:
Total investments	135,505,000	127,172,000
Money market funds
Financial Assets:
Total investments	83,508,000	102,430,000
U.S. Agency | U.S. agency securities
Financial Assets:
Total investments	26,357,000	18,441,000
U.S. Agency | U.S. agency mortgage-backed securities
Financial Assets:
Total investments	397,602,000	316,494,000
Recurring basis
Financial Assets:
Assets fair value	1,841,041,000	1,615,102,000
Recurring basis | U.S. Treasury securities
Financial Assets:
Total investments	202,364,000	191,548,000
Recurring basis | Municipal debt securities
Financial Assets:
Total investments	370,068,000	334,324,000
Recurring basis | Corporate debt securities
Financial Assets:
Total investments	555,965,000	456,357,000
Recurring basis | Residential and commercial mortgage securities
Financial Assets:
Total investments	69,672,000	68,336,000
Recurring basis | Asset-backed securities
Financial Assets:
Total investments	135,505,000	127,172,000
Recurring basis | Money market funds
Financial Assets:
Total investments	83,508,000	102,430,000
Recurring basis | U.S. Agency | U.S. agency securities
Financial Assets:
Total investments	26,357,000	18,441,000
Recurring basis | U.S. Agency | U.S. agency mortgage-backed securities
Financial Assets:
Total investments	397,602,000	316,494,000
Quoted Prices in Active Markets for Identical Instruments (Level 1) | Recurring basis
Financial Assets:
Assets fair value	285,872,000	293,978,000
Quoted Prices in Active Markets for Identical Instruments (Level 1) | Recurring basis | U.S. Treasury securities
Financial Assets:
Total investments	202,364,000	191,548,000
Quoted Prices in Active Markets for Identical Instruments (Level 1) | Recurring basis | Municipal debt securities
Financial Assets:
Total investments	0	0
Quoted Prices in Active Markets for Identical Instruments (Level 1) | Recurring basis | Corporate debt securities
Financial Assets:
Total investments	0	0
Quoted Prices in Active Markets for Identical Instruments (Level 1) | Recurring basis | Residential and commercial mortgage securities
Financial Assets:
Total investments	0	0
Quoted Prices in Active Markets for Identical Instruments (Level 1) | Recurring basis | Asset-backed securities
Financial Assets:
Total investments	0	0
Quoted Prices in Active Markets for Identical Instruments (Level 1) | Recurring basis | Money market funds
Financial Assets:
Total investments	83,508,000	102,430,000
Quoted Prices in Active Markets for Identical Instruments (Level 1) | Recurring basis | U.S. Agency | U.S. agency securities
Financial Assets:
Total investments	0	0
Quoted Prices in Active Markets for Identical Instruments (Level 1) | Recurring basis | U.S. Agency | U.S. agency mortgage-backed securities
Financial Assets:
Total investments	0	0
Significant Other Observable Inputs (Level 2) | Recurring basis
Financial Assets:
Assets fair value	1,555,169,000	1,321,124,000
Significant Other Observable Inputs (Level 2) | Recurring basis | U.S. Treasury securities
Financial Assets:
Total investments	0	0
Significant Other Observable Inputs (Level 2) | Recurring basis | Municipal debt securities
Financial Assets:
Total investments	370,068,000	334,324,000
Significant Other Observable Inputs (Level 2) | Recurring basis | Corporate debt securities
Financial Assets:
Total investments	555,965,000	456,357,000
Significant Other Observable Inputs (Level 2) | Recurring basis | Residential and commercial mortgage securities
Financial Assets:
Total investments	69,672,000	68,336,000
Significant Other Observable Inputs (Level 2) | Recurring basis | Asset-backed securities
Financial Assets:
Total investments	135,505,000	127,172,000
Significant Other Observable Inputs (Level 2) | Recurring basis | Money market funds
Financial Assets:
Total investments	0	0
Significant Other Observable Inputs (Level 2) | Recurring basis | U.S. Agency | U.S. agency securities
Financial Assets:
Total investments	26,357,000	18,441,000
Significant Other Observable Inputs (Level 2) | Recurring basis | U.S. Agency | U.S. agency mortgage-backed securities
Financial Assets:
Total investments	397,602,000	316,494,000
Significant Unobservable Inputs (Level 3) | Recurring basis
Financial Assets:
Assets fair value	0	0
Significant Unobservable Inputs (Level 3) | Recurring basis | U.S. Treasury securities
Financial Assets:
Total investments	0	0
Significant Unobservable Inputs (Level 3) | Recurring basis | Municipal debt securities
Financial Assets:
Total investments	0	0
Significant Unobservable Inputs (Level 3) | Recurring basis | Corporate debt securities
Financial Assets:
Total investments	0	0
Significant Unobservable Inputs (Level 3) | Recurring basis | Residential and commercial mortgage securities
Financial Assets:
Total investments	0	0
Significant Unobservable Inputs (Level 3) | Recurring basis | Asset-backed securities
Financial Assets:
Total investments	0	0
Significant Unobservable Inputs (Level 3) | Recurring basis | Money market funds
Financial Assets:
Total investments	0	0
Significant Unobservable Inputs (Level 3) | Recurring basis | U.S. Agency | U.S. agency securities
Financial Assets:
Total investments	0	0
Significant Unobservable Inputs (Level 3) | Recurring basis | U.S. Agency | U.S. agency mortgage-backed securities
Financial Assets:
Total investments	$ 0	$ 0