Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Financial Assets:
Total investments	$ 1,368,502	$ 1,276,634
U.S. Treasury securities
Financial Assets:
Total investments	211,780	177,607
U.S. agency securities
Financial Assets:
Total investments	14,657	13,782
Municipal debt securities
Financial Assets:
Total investments	304,111	279,828
Corporate debt securities
Financial Assets:
Total investments	416,688	396,732
Mortgage-backed securities
Financial Assets:
Total investments	53,046	55,356
Asset-backed securities
Financial Assets:
Total investments	121,729	126,629
Money market funds
Financial Assets:
Total investments	68,488	67,098
U.S. Agency | Mortgage backed securities
Financial Assets:
Total investments	178,003	159,602
Carrying Amount
Financial Assets:
Total investments	1,368,502	1,276,634
Financial Liabilities:
Derivative liabilities	898	1,232
Carrying Amount | U.S. Treasury securities
Financial Assets:
Total investments	211,780	177,607
Carrying Amount | U.S. agency securities
Financial Assets:
Total investments	14,657	13,782
Carrying Amount | Municipal debt securities
Financial Assets:
Total investments	304,111	279,828
Carrying Amount | Corporate debt securities
Financial Assets:
Total investments	416,688	396,732
Carrying Amount | Mortgage-backed securities
Financial Assets:
Total investments	53,046	55,356
Carrying Amount | Asset-backed securities
Financial Assets:
Total investments	121,729	126,629
Carrying Amount | Money market funds
Financial Assets:
Total investments	68,488	67,098
Carrying Amount | U.S. Agency | Mortgage backed securities
Financial Assets:
Total investments	$ 178,003	$ 159,602