Investments - Summary of Amortized Cost and Estimated Fair Value of Investments Excluding VIE Investments by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Due in one year or less	$ 855,714
Amortized Cost, Due after one year through five years	1,020,421
Amortized Cost, Due after five years through ten years	1,013,742
Amortized Cost, Due after ten years	972,316
Amortized Cost, Total	3,862,193
Amortized Cost	5,678,660	6,391,970
Estimated Fair Value, Due in one year or less	857,197
Estimated Fair Value, Due after one year through five years	1,070,411
Estimated Fair Value, Due after five years through ten years	1,135,469
Estimated Fair Value, Due after ten years	1,071,294
Estimated Fair Value due, Total	4,134,371
Estimated Fair Value	6,329,932	6,876,990
Residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,096,202
Estimated Fair Value	1,455,582
Collateralized debt obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	32,855
Estimated Fair Value	33,342
Other asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	687,410
Estimated Fair Value	$ 706,637