Contractual Maturity by Number of Years Until Maturity for Fixed Maturity Securities with Gross Unrealized Loss (Detail) (Fixed maturities, USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	$ 2,784	$ 1,261
Percent of Total Gross Unrealized Loss	100.00%
Fair Value Amount	256,301	156,071
Percent of Total Fair Value	100.00%
Due in one year or less
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	3
Percent of Total Gross Unrealized Loss	0.00%
Fair Value Amount	1,505
Percent of Total Fair Value	0.00%
Due After One Year Through Five Years
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	63
Percent of Total Gross Unrealized Loss	2.00%
Fair Value Amount	26,313
Percent of Total Fair Value	10.00%
Due After Five Years Through Ten Years
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	446
Percent of Total Gross Unrealized Loss	16.00%
Fair Value Amount	76,159
Percent of Total Fair Value	30.00%
Due After Ten Years
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	1,387
Percent of Total Gross Unrealized Loss	50.00%
Fair Value Amount	86,381
Percent of Total Fair Value	34.00%
Mortgage- and asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	885
Percent of Total Gross Unrealized Loss	32.00%
Fair Value Amount	$ 65,943
Percent of Total Fair Value	26.00%