Contractual Maturity by Number of Years Until Maturity for Fixed Maturity Securities with a Gross Unrealized Loss (Detail) (Fixed maturities, USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	$ 10,782	$ 20,762
Percent of Total Gross Unrealized Loss	100.00%
Fair Value Amount	305,121	757,305
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	8
Percent of Total Gross Unrealized Loss	0.00%
Fair Value Amount	5,570
Percent of Total Fair Value	2.00%
Due after one year through five years
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	4,992
Percent of Total Gross Unrealized Loss	46.00%
Fair Value Amount	190,234
Percent of Total Fair Value	62.00%
Due after five years through ten years
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	1,786
Percent of Total Gross Unrealized Loss	17.00%
Fair Value Amount	42,649
Percent of Total Fair Value	14.00%
Due After Ten Years
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	477
Percent of Total Gross Unrealized Loss	4.00%
Fair Value Amount	13,522
Percent of Total Fair Value	4.00%
Mortgage- and asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	3,519
Percent of Total Gross Unrealized Loss	33.00%
Fair Value Amount	$ 53,146
Percent of Total Fair Value	18.00%