Fair Value of Financial Instruments (Details 2) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Fair value of financial instruments
Total investments	$ 1,276,634,000	$ 1,057,613,000
U.S. Treasury securities
Fair value of financial instruments
Total investments	177,607,000	74,216,000
U.S. agency securities
Fair value of financial instruments
Total investments	13,782,000	4,520,000
Municipal debt securities
Fair value of financial instruments
Total investments	279,828,000	195,546,000
Corporate debt securities
Fair value of financial instruments
Total investments	396,732,000	296,829,000
Mortgage-backed securities
Fair value of financial instruments
Total investments	55,356,000	66,086,000
Asset-backed securities
Fair value of financial instruments
Total investments	126,629,000	126,188,000
Money market funds
Fair value of financial instruments
Total investments	67,098,000	210,688,000
U.S. Agency \| U.S. agency mortgage-backed securities
Fair value of financial instruments
Total investments	159,602,000	83,540,000
Recurring basis
Fair value of financial instruments
Total assets at fair value	1,276,634,000	1,057,613,000
Derivative liabilities	1,232,000	661,000
Total liabilities at fair value	1,232,000	661,000
Recurring basis \| U.S. Treasury securities
Fair value of financial instruments
Total investments	177,607,000	74,216,000
Recurring basis \| U.S. agency securities
Fair value of financial instruments
Total investments	13,782,000	4,520,000
Recurring basis \| Municipal debt securities
Fair value of financial instruments
Total investments	279,828,000	195,546,000
Recurring basis \| Corporate debt securities
Fair value of financial instruments
Total investments	396,732,000	296,829,000
Recurring basis \| Mortgage-backed securities
Fair value of financial instruments
Total investments	55,356,000	66,086,000
Recurring basis \| Asset-backed securities
Fair value of financial instruments
Total investments	126,629,000	126,188,000
Recurring basis \| Money market funds
Fair value of financial instruments
Total investments	67,098,000	210,688,000
Recurring basis \| U.S. Agency \| U.S. agency mortgage-backed securities
Fair value of financial instruments
Total investments	159,602,000	83,540,000
Recurring basis \| Quoted Prices in Active Markets for Identical Instruments (Level 1)
Fair value of financial instruments
Total assets at fair value	244,705,000	284,904,000
Recurring basis \| Quoted Prices in Active Markets for Identical Instruments (Level 1) \| U.S. Treasury securities
Fair value of financial instruments
Total investments	177,607,000	74,216,000
Recurring basis \| Quoted Prices in Active Markets for Identical Instruments (Level 1) \| Money market funds
Fair value of financial instruments
Total investments	67,098,000	210,688,000
Recurring basis \| Significant Other Observable Inputs (Level 2)
Fair value of financial instruments
Total assets at fair value	1,031,929,000	772,709,000
Recurring basis \| Significant Other Observable Inputs (Level 2) \| U.S. agency securities
Fair value of financial instruments
Total investments	13,782,000	4,520,000
Recurring basis \| Significant Other Observable Inputs (Level 2) \| Municipal debt securities
Fair value of financial instruments
Total investments	279,828,000	195,546,000
Recurring basis \| Significant Other Observable Inputs (Level 2) \| Corporate debt securities
Fair value of financial instruments
Total investments	396,732,000	296,829,000
Recurring basis \| Significant Other Observable Inputs (Level 2) \| Mortgage-backed securities
Fair value of financial instruments
Total investments	55,356,000	66,086,000
Recurring basis \| Significant Other Observable Inputs (Level 2) \| Asset-backed securities
Fair value of financial instruments
Total investments	126,629,000	126,188,000
Recurring basis \| Significant Other Observable Inputs (Level 2) \| U.S. Agency \| U.S. agency mortgage-backed securities
Fair value of financial instruments
Total investments	159,602,000	83,540,000
Recurring basis \| Significant Unobservable Inputs (Level 3)
Fair value of financial instruments
Total assets at fair value	0	0
Total liabilities at fair value	1,232,000	661,000
Derivative liabilities \| Recurring basis \| Significant Unobservable Inputs (Level 3)
Fair value of financial instruments
Derivative liabilities	$ 1,232,000	$ 661,000

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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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