Investments - Summary of Amortized Cost and Estimated Fair Value of Investments Excluding VIE Investments by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Amortized cost due in one year or less	$ 896,968
Amortized cost due after one year through five years	881,664
Amortized cost due after five years through ten years	1,099,250
Amortized cost due after ten years	1,098,182
Estimated fair value due in one year or less	899,698
Estimated fair value due after one year through five years	933,801
Estimated fair value due after five years through ten years	1,214,631
Estimated fair value due after ten years	1,201,159
Amortized cost of investments	5,886,205	6,391,970
Estimated fair value on investments	6,440,117	6,876,990
Residential Mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,144,258
Estimated Fair Value	1,409,026
Collateralized debt obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	36,814
Estimated Fair Value	36,809
Other Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	729,069
Estimated Fair Value	$ 744,993