Summary of Amortized Cost and Estimated Fair Value Through Fixed Maturities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	$ 1,380,323	$ 1,418,073
Fixed Maturities, estimated fair value	1,457,335	1,471,863
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	97,197
Due after one year through five years, Amortized Cost	563,927
Due after five years through ten years, Amortized Cost	125,672
Due after ten years through fifteen years, Amortized Cost	9,609
Due after fifteen years, Amortized Cost	33,234
Fixed maturities, amortized cost	1,212,179	1,258,533
Due in one year or less, Estimated Fair value	98,377
Due after one year through five years, Estimated Fair value	591,407
Due after five years through ten years, Estimated Fair value	130,332
Due after ten years through fifteen years, Estimated Fair value	11,099
Due after fifteen years, Estimated Fair value	33,598
Fixed Maturities, estimated fair value	1,259,845	1,296,885
Fixed Maturities [Member] | Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	263,406
Fixed maturities, amortized cost	263,406	259,935
Estimated Fair value	273,550
Fixed Maturities, estimated fair value	273,550	268,990
Fixed Maturities [Member] | Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	106,513
Fixed maturities, amortized cost	106,513	94,096
Estimated Fair value	108,816
Fixed Maturities, estimated fair value	108,816	95,964
Fixed Maturities [Member] | Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,621
Fixed maturities, amortized cost	12,621	29,975
Estimated Fair value	12,666
Fixed Maturities, estimated fair value	$ 12,666	$ 29,969