Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Financial Assets:
Total investments	$ 845,870	$ 332,555
Financial Liabilities:
Amounts due under Asset Purchase Agreement	2,483	4,949
U.S. Treasury securities
Financial Assets:
Total investments	66,719	59,187
U.S. agency securities
Financial Assets:
Total investments	8,165	14,839
Municipal debt securities
Financial Assets:
Total investments	170,795	57,650
Corporate debt securities
Financial Assets:
Total investments	241,874	125,593
Mortgage-backed securities
Financial Assets:
Total investments	54,207	18,581
Asset backed securities
Financial Assets:
Total investments	88,710	20,385
Money market funds
Financial Assets:
Total investments	154,472	14,079
U.S. agency | Mortgage-backed securities
Financial Assets:
Total investments	60,928	22,241
Carrying Amount
Financial Assets:
Total investments	845,870	332,555
Financial Liabilities:
Amounts due under Asset Purchase Agreement	2,483	4,949
Carrying Amount | U.S. Treasury securities
Financial Assets:
Total investments	66,719	59,187
Carrying Amount | U.S. agency securities
Financial Assets:
Total investments	8,165	14,839
Carrying Amount | Municipal debt securities
Financial Assets:
Total investments	170,795	57,650
Carrying Amount | Corporate debt securities
Financial Assets:
Total investments	241,874	125,593
Carrying Amount | Mortgage-backed securities
Financial Assets:
Total investments	54,207	18,581
Carrying Amount | Asset backed securities
Financial Assets:
Total investments	88,710	20,385
Carrying Amount | Money market funds
Financial Assets:
Total investments	154,472	14,079
Carrying Amount | U.S. agency | Mortgage-backed securities
Financial Assets:
Total investments	60,928	22,241
Fair Value
Financial Assets:
Total investments	845,870	332,555
Financial Liabilities:
Amounts due under Asset Purchase Agreement	2,499	4,997
Fair Value | U.S. Treasury securities
Financial Assets:
Total investments	66,719	59,187
Fair Value | U.S. agency securities
Financial Assets:
Total investments	8,165	14,839
Fair Value | Municipal debt securities
Financial Assets:
Total investments	170,795	57,650
Fair Value | Corporate debt securities
Financial Assets:
Total investments	241,874	125,593
Fair Value | Mortgage-backed securities
Financial Assets:
Total investments	54,207	18,581
Fair Value | Asset backed securities
Financial Assets:
Total investments	88,710	20,385
Fair Value | Money market funds
Financial Assets:
Total investments	154,472	14,079
Fair Value | U.S. agency | Mortgage-backed securities
Financial Assets:
Total investments	$ 60,928	$ 22,241