Investment Securities - Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Estimated Fair Values of Fixed-Maturity Securities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total fixed- maturity securities, Amortized Cost	$ 272,895	$ 298,978
Total fixed-maturity securities, Gross Unrealized Gains	4,869	6,442
Total fixed-maturity securities, Gross Unrealized Losses	(295)	(1,375)
Total fixed-maturity securities, Estimated Fair Value	277,469	304,045
Less total unrestricted fixed-maturity securities, Amortized Cost	75,659	5,348
Less total unrestricted fixed-maturity securities, Gross Unrealized Gains	168	151
Less total unrestricted fixed-maturity securities, Gross Unrealized Losses	(18)	(9)
Less total unrestricted fixed-maturity securities, Estimated Fair Value	75,809	5,490
Total fixed-maturity securities held in trust, Amortized Cost	197,236
Total fixed-maturity securities held in trust, Gross Unrealized Gains	4,701
Total fixed-maturity securities held in trust, Gross Unrealized Losses	(277)
Total fixed-maturity securities held in trust, Estimated Fair Value	201,660
Obligations of U.S. government sponsored enterprises [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total fixed- maturity securities, Amortized Cost	97,103	122,874
Total fixed-maturity securities, Gross Unrealized Gains	1,416	2,029
Total fixed-maturity securities, Gross Unrealized Losses	(156)	(267)
Total fixed-maturity securities, Estimated Fair Value	98,363	124,636
Corporate debt securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total fixed- maturity securities, Amortized Cost	91,997	129,916
Total fixed-maturity securities, Gross Unrealized Gains	3,010	3,791
Total fixed-maturity securities, Gross Unrealized Losses	(34)	(845)
Total fixed-maturity securities, Estimated Fair Value	94,973	132,862
Commercial paper [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total fixed- maturity securities, Amortized Cost	54,949
Total fixed-maturity securities, Estimated Fair Value	54,949
Foreign corporate debt securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total fixed- maturity securities, Amortized Cost	18,305	26,800
Total fixed-maturity securities, Gross Unrealized Gains	379	546
Total fixed-maturity securities, Gross Unrealized Losses	(18)	(117)
Total fixed-maturity securities, Estimated Fair Value	18,666	27,229
Commercial Mortgage-backed securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total fixed- maturity securities, Amortized Cost	10,541	19,138
Total fixed-maturity securities, Gross Unrealized Gains	64	76
Total fixed-maturity securities, Gross Unrealized Losses	(87)	(146)
Total fixed-maturity securities, Estimated Fair Value	10,518	19,068
U.S. Treasury securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total fixed- maturity securities, Amortized Cost		250
Total fixed-maturity securities, Estimated Fair Value		$ 250