Investment Securities - Schedule of Investments in Accordance with Fair Value Measurement (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Class of Security:
Total fixed maturity securities	$ 250,300	$ 277,469
Total investments fair value	270,856	311,097
Obligations of U.S. Government Sponsored Enterprises [Member]
Class of Security:
Total fixed maturity securities	57,593	98,363
Corporate Debt Securities [Member]
Class of Security:
Total fixed maturity securities	103,562	94,974
Commercial Paper [Member]
Class of Security:
Total fixed maturity securities	59,978	54,948
Foreign Corporate Debt Securities [Member]
Class of Security:
Total fixed maturity securities	26,835	18,666
Commercial Mortgage-Backed Securities [Member]
Class of Security:
Total fixed maturity securities	2,332	10,518
Money Market Funds [Member]
Class of Security:
Total investments fair value	20,556	33,628
Fair Value, Inputs, Level 1 [Member]
Class of Security:
Total fixed maturity securities	89,262	90,144
Total investments fair value	109,818	123,772
Fair Value, Inputs, Level 1 [Member] | Obligations of U.S. Government Sponsored Enterprises [Member]
Class of Security:
Total fixed maturity securities	7,438	11,362
Fair Value, Inputs, Level 1 [Member] | Corporate Debt Securities [Member]
Class of Security:
Total fixed maturity securities	19,285	22,833
Fair Value, Inputs, Level 1 [Member] | Commercial Paper [Member]
Class of Security:
Total fixed maturity securities	59,978	54,948
Fair Value, Inputs, Level 1 [Member] | Foreign Corporate Debt Securities [Member]
Class of Security:
Total fixed maturity securities	2,561	1,001
Fair Value, Inputs, Level 1 [Member] | Commercial Mortgage-Backed Securities [Member]
Class of Security:
Total fixed maturity securities
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Class of Security:
Total investments fair value	20,556	33,628
Fair Value, Inputs, Level 2 [Member]
Class of Security:
Total fixed maturity securities	161,038	187,325
Total investments fair value	161,038	187,325
Fair Value, Inputs, Level 2 [Member] | Obligations of U.S. Government Sponsored Enterprises [Member]
Class of Security:
Total fixed maturity securities	50,155	87,001
Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Class of Security:
Total fixed maturity securities	84,277	72,141
Fair Value, Inputs, Level 2 [Member] | Commercial Paper [Member]
Class of Security:
Total fixed maturity securities
Fair Value, Inputs, Level 2 [Member] | Foreign Corporate Debt Securities [Member]
Class of Security:
Total fixed maturity securities	24,274	17,665
Fair Value, Inputs, Level 2 [Member] | Commercial Mortgage-Backed Securities [Member]
Class of Security:
Total fixed maturity securities	2,332	10,518
Fair Value, Inputs, Level 2 [Member] | Money Market Funds [Member]
Class of Security:
Total investments fair value
Fair Value, Inputs, Level 3 [Member]
Class of Security:
Total fixed maturity securities
Total investments fair value
Fair Value, Inputs, Level 3 [Member] | Obligations of U.S. Government Sponsored Enterprises [Member]
Class of Security:
Total fixed maturity securities
Fair Value, Inputs, Level 3 [Member] | Corporate Debt Securities [Member]
Class of Security:
Total fixed maturity securities
Fair Value, Inputs, Level 3 [Member] | Commercial Paper [Member]
Class of Security:
Total fixed maturity securities
Fair Value, Inputs, Level 3 [Member] | Foreign Corporate Debt Securities [Member]
Class of Security:
Total fixed maturity securities
Fair Value, Inputs, Level 3 [Member] | Commercial Mortgage-Backed Securities [Member]
Class of Security:
Total fixed maturity securities
Fair Value, Inputs, Level 3 [Member] | Money Market Funds [Member]
Class of Security:
Total investments fair value