Schedule of Investments in Accordance with Fair Value Measurement (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Class of Security:
Commercial mortgage-backed securities	$ 281,294	$ 304,045
US Treasury Securities [Member]
Class of Security:
Commercial mortgage-backed securities	250	250
US Government-sponsored Enterprises Debt Securities [Member]
Class of Security:
Commercial mortgage-backed securities	102,908	124,636
Corporate Debt Securities [Member]
Class of Security:
Commercial mortgage-backed securities	102,876	132,862
Commercial Paper [Member]
Class of Security:
Commercial mortgage-backed securities	44,915
Foreign Corporate Debt Securities [Member]
Class of Security:
Commercial mortgage-backed securities	18,784	27,229
Commercial Mortgage Backed Securities [Member]
Class of Security:
Commercial mortgage-backed securities	11,561	19,068
Fair Value, Inputs, Level 1 [Member]
Class of Security:
Commercial mortgage-backed securities	61,580	43,177
Fair Value, Inputs, Level 1 [Member] | US Treasury Securities [Member]
Class of Security:
Commercial mortgage-backed securities	250	250
Fair Value, Inputs, Level 1 [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Class of Security:
Commercial mortgage-backed securities	16,415	42,927
Fair Value, Inputs, Level 1 [Member] | Commercial Paper [Member]
Class of Security:
Commercial mortgage-backed securities	44,915
Fair Value, Inputs, Level 2 [Member]
Class of Security:
Commercial mortgage-backed securities	219,714	260,868
Fair Value, Inputs, Level 2 [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Class of Security:
Commercial mortgage-backed securities	86,493	81,709
Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Class of Security:
Commercial mortgage-backed securities	102,876	132,862
Fair Value, Inputs, Level 2 [Member] | Foreign Corporate Debt Securities [Member]
Class of Security:
Commercial mortgage-backed securities	18,784	27,229
Fair Value, Inputs, Level 2 [Member] | Commercial Mortgage Backed Securities [Member]
Class of Security:
Commercial mortgage-backed securities	$ 11,561	$ 19,068