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	Mar. 31, 2013	Dec. 31, 2012
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total fixed- maturity securities, Amortized Cost	$ 277,654	$ 272,895
Total fixed-maturity securities, Gross Unrealized Gains	4,252	4,869
Total fixed-maturity securities, Gross Unrealized Losses	(359)	(295)
Total fixed-maturity securities, Estimated Fair Value	281,547	277,469
Total fixed-maturity securities held in trust, Amortized Cost	194,196	197,236
Total fixed-maturity securities held in trust, Gross Unrealized Gains	4,078	4,701
Total fixed-maturity securities held in trust, Gross Unrealized Losses	(326)	(277)
Total fixed-maturity securities held in trust, Estimated Fair Value	197,948	201,660
Trading [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Less total unrestricted fixed-maturity securities, Amortized Cost	10,002	14,246
Less total unrestricted fixed-maturity securities, Gross Unrealized Gains	174	168
Less total unrestricted fixed-maturity securities, Gross Unrealized Losses	(33)	(18)
Less total unrestricted fixed-maturity securities, Estimated Fair Value	10,143	14,396
Held to maturity [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Less total unrestricted fixed-maturity securities, Amortized Cost	73,456	61,413
Less total unrestricted fixed-maturity securities, Estimated Fair Value	73,456	61,413
Obligations of U.S. government sponsored enterprises [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total fixed- maturity securities, Amortized Cost	73,055	97,104
Total fixed-maturity securities, Gross Unrealized Gains	1,040	1,415
Total fixed-maturity securities, Gross Unrealized Losses	(199)	(156)
Total fixed-maturity securities, Estimated Fair Value	73,896	98,363
Corporate debt securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total fixed- maturity securities, Amortized Cost	105,320	91,997
Total fixed-maturity securities, Gross Unrealized Gains	2,853	3,011
Total fixed-maturity securities, Gross Unrealized Losses	(59)	(34)
Total fixed-maturity securities, Estimated Fair Value	108,114	94,974
Commercial paper [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total fixed- maturity securities, Amortized Cost	69,607	54,948
Total fixed-maturity securities, Estimated Fair Value	69,607	54,948
Foreign corporate debt securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total fixed- maturity securities, Amortized Cost	20,779	18,305
Total fixed-maturity securities, Gross Unrealized Gains	326	379
Total fixed-maturity securities, Gross Unrealized Losses	(23)	(18)
Total fixed-maturity securities, Estimated Fair Value	21,082	18,666
Commercial Mortgage-backed securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total fixed- maturity securities, Amortized Cost	8,893	10,541
Total fixed-maturity securities, Gross Unrealized Gains	33	64
Total fixed-maturity securities, Gross Unrealized Losses	(78)	(87)
Total fixed-maturity securities, Estimated Fair Value	$ 8,848	$ 10,518