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	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	$ 29,261	$ 1,261
Percent of Total Gross Unrealized Loss	100.00%
Fair Value Amount	812,704	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	12
Percent of Total Gross Unrealized Loss	0.00%
Fair Value Amount	11,813
Percent of Total Fair Value	1.00%
Due After One Year Through Five Years
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	2,442
Percent of Total Gross Unrealized Loss	8.00%
Fair Value Amount	192,689
Percent of Total Fair Value	24.00%
Due After Five Years Through Ten Years
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	11,394
Percent of Total Gross Unrealized Loss	39.00%
Fair Value Amount	267,617
Percent of Total Fair Value	33.00%
Due After Ten Years
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	9,920
Percent of Total Gross Unrealized Loss	34.00%
Fair Value Amount	169,833
Percent of Total Fair Value	21.00%
Mortgage- and asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	5,493
Percent of Total Gross Unrealized Loss	19.00%
Fair Value Amount	$ 170,752
Percent of Total Fair Value	21.00%