Fair Value of Financial Instruments (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Fair value of financial instruments
Total investments	$ 1,615,102,000	$ 1,276,634,000
U.S. Treasury securities
Fair value of financial instruments
Total investments	191,548,000	177,607,000
U.S. agency securities
Fair value of financial instruments
Total investments	18,441,000	13,782,000
Municipal debt securities
Fair value of financial instruments
Total investments	334,324,000	279,828,000
Corporate debt securities
Fair value of financial instruments
Total investments	456,357,000	396,732,000
Residential and commercial mortgage securities
Fair value of financial instruments
Total investments	68,336,000	55,356,000
Asset-backed securities
Fair value of financial instruments
Total investments	127,172,000	126,629,000
Money market funds
Fair value of financial instruments
Total investments	102,430,000	67,098,000
U.S. Agency \| U.S. agency mortgage-backed securities
Fair value of financial instruments
Total investments	316,494,000	159,602,000
Recurring basis
Fair value of financial instruments
Total assets at fair value	1,615,102,000	1,276,634,000
Derivative liabilities	0	1,232,000
Total liabilities at fair value	0	1,232,000
Recurring basis \| U.S. Treasury securities
Fair value of financial instruments
Total investments	191,548,000	177,607,000
Recurring basis \| U.S. agency securities
Fair value of financial instruments
Total investments	18,441,000	13,782,000
Recurring basis \| Municipal debt securities
Fair value of financial instruments
Total investments	334,324,000	279,828,000
Recurring basis \| Corporate debt securities
Fair value of financial instruments
Total investments	456,357,000	396,732,000
Recurring basis \| Residential and commercial mortgage securities
Fair value of financial instruments
Total investments	68,336,000	55,356,000
Recurring basis \| Asset-backed securities
Fair value of financial instruments
Total investments	127,172,000	126,629,000
Recurring basis \| Money market funds
Fair value of financial instruments
Total investments	102,430,000	67,098,000
Recurring basis \| U.S. Agency \| U.S. agency mortgage-backed securities
Fair value of financial instruments
Total investments	316,494,000	159,602,000
Recurring basis \| Quoted Prices in Active Markets for Identical Instruments (Level 1)
Fair value of financial instruments
Total assets at fair value	293,978,000	244,705,000
Recurring basis \| Quoted Prices in Active Markets for Identical Instruments (Level 1) \| U.S. Treasury securities
Fair value of financial instruments
Total investments	191,548,000	177,607,000
Recurring basis \| Quoted Prices in Active Markets for Identical Instruments (Level 1) \| Money market funds
Fair value of financial instruments
Total investments	102,430,000	67,098,000
Recurring basis \| Significant Other Observable Inputs (Level 2)
Fair value of financial instruments
Total assets at fair value	1,321,124,000	1,031,929,000
Recurring basis \| Significant Other Observable Inputs (Level 2) \| U.S. agency securities
Fair value of financial instruments
Total investments	18,441,000	13,782,000
Recurring basis \| Significant Other Observable Inputs (Level 2) \| Municipal debt securities
Fair value of financial instruments
Total investments	334,324,000	279,828,000
Recurring basis \| Significant Other Observable Inputs (Level 2) \| Corporate debt securities
Fair value of financial instruments
Total investments	456,357,000	396,732,000
Recurring basis \| Significant Other Observable Inputs (Level 2) \| Residential and commercial mortgage securities
Fair value of financial instruments
Total investments	68,336,000	55,356,000
Recurring basis \| Significant Other Observable Inputs (Level 2) \| Asset-backed securities
Fair value of financial instruments
Total investments	127,172,000	126,629,000
Recurring basis \| Significant Other Observable Inputs (Level 2) \| U.S. Agency \| U.S. agency mortgage-backed securities
Fair value of financial instruments
Total investments	316,494,000	159,602,000
Recurring basis \| Significant Unobservable Inputs (Level 3)
Fair value of financial instruments
Total assets at fair value	0	0
Derivative liabilities	0	1,232,000
Total liabilities at fair value	$ 0	$ 1,232,000

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Fair value of financial obligations, including, but not limited to, debt instruments, derivative liabilities, federal funds purchased and sold under agreements to repurchase, securities loaned or sold under agreements to repurchase, financial instruments sold not yet purchased, guarantees, line of credit, loans and notes payable, servicing liability, and trading liabilities.

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