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	Sep. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	$ 25,496	$ 1,261
Percent of Total Gross Unrealized Loss	100.00%
Fair Value Amount	731,167	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	1
Percent of Total Gross Unrealized Loss	0.00%
Fair Value Amount	1,716
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	1,119
Percent of Total Gross Unrealized Loss	4.00%
Fair Value Amount	144,849
Percent of Total Fair Value	20.00%
Due After Five Years Through Ten Years
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	9,803
Percent of Total Gross Unrealized Loss	38.00%
Fair Value Amount	250,290
Percent of Total Fair Value	34.00%
Due After Ten Years
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	9,869
Percent of Total Gross Unrealized Loss	40.00%
Fair Value Amount	162,191
Percent of Total Fair Value	22.00%
Mortgage- and asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	4,704
Percent of Total Gross Unrealized Loss	18.00%
Fair Value Amount	$ 172,121
Percent of Total Fair Value	24.00%