Investment Securities - Schedule of Investments in Accordance with Fair Value Measurement (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Class of Security:
Commercial mortgage-backed securities	$ 277,469	$ 304,045
U.S. Treasury securities [Member]
Class of Security:
Commercial mortgage-backed securities		250
Obligations of U.S. government sponsored enterprises [Member]
Class of Security:
Commercial mortgage-backed securities	98,363	124,636
Corporate debt securities [Member]
Class of Security:
Commercial mortgage-backed securities	94,974	132,862
Foreign corporate debt securities [Member]
Class of Security:
Commercial mortgage-backed securities	18,666	27,229
Commercial Mortgage-backed securities [Member]
Class of Security:
Commercial mortgage-backed securities	10,518	19,068
Commercial paper [Member]
Class of Security:
Commercial mortgage-backed securities	54,948
Fair Value, Inputs, Level 1 [Member]
Class of Security:
Commercial mortgage-backed securities	90,144	43,177
Fair Value, Inputs, Level 1 [Member] | U.S. Treasury securities [Member]
Class of Security:
Commercial mortgage-backed securities		250
Fair Value, Inputs, Level 1 [Member] | Obligations of U.S. government sponsored enterprises [Member]
Class of Security:
Commercial mortgage-backed securities	11,362	42,927
Fair Value, Inputs, Level 1 [Member] | Corporate debt securities [Member]
Class of Security:
Commercial mortgage-backed securities	22,833
Fair Value, Inputs, Level 1 [Member] | Foreign corporate debt securities [Member]
Class of Security:
Commercial mortgage-backed securities	1,001
Fair Value, Inputs, Level 1 [Member] | Commercial Mortgage-backed securities [Member]
Class of Security:
Commercial mortgage-backed securities
Fair Value, Inputs, Level 1 [Member] | Commercial paper [Member]
Class of Security:
Commercial mortgage-backed securities	54,948
Fair Value, Inputs, Level 2 [Member]
Class of Security:
Commercial mortgage-backed securities	187,325	260,868
Fair Value, Inputs, Level 2 [Member] | Obligations of U.S. government sponsored enterprises [Member]
Class of Security:
Commercial mortgage-backed securities	87,001	81,709
Fair Value, Inputs, Level 2 [Member] | Corporate debt securities [Member]
Class of Security:
Commercial mortgage-backed securities	72,141	132,862
Fair Value, Inputs, Level 2 [Member] | Foreign corporate debt securities [Member]
Class of Security:
Commercial mortgage-backed securities	17,665	27,229
Fair Value, Inputs, Level 2 [Member] | Commercial Mortgage-backed securities [Member]
Class of Security:
Commercial mortgage-backed securities	10,518	19,068
Fair Value, Inputs, Level 3 [Member]
Class of Security:
Commercial mortgage-backed securities
Fair Value, Inputs, Level 3 [Member] | U.S. Treasury securities [Member]
Class of Security:
Commercial mortgage-backed securities
Fair Value, Inputs, Level 3 [Member] | Obligations of U.S. government sponsored enterprises [Member]
Class of Security:
Commercial mortgage-backed securities
Fair Value, Inputs, Level 3 [Member] | Corporate debt securities [Member]
Class of Security:
Commercial mortgage-backed securities
Fair Value, Inputs, Level 3 [Member] | Foreign corporate debt securities [Member]
Class of Security:
Commercial mortgage-backed securities
Fair Value, Inputs, Level 3 [Member] | Commercial Mortgage-backed securities [Member]
Class of Security:
Commercial mortgage-backed securities
Fair Value, Inputs, Level 3 [Member] | Commercial paper [Member]
Class of Security:
Commercial mortgage-backed securities