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	Sep. 30, 2011	Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	$ 11,927	$ 20,762
Percent of Total Gross Unrealized Loss	100.00%
Fair Value Amount	402,244	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	3
Percent of Total Gross Unrealized Loss	0.00%
Fair Value Amount	4,509
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	5,229
Percent of Total Gross Unrealized Loss	44.00%
Fair Value Amount	204,736
Percent of Total Fair Value	51.00%
Due after five years through ten years
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	2,592
Percent of Total Gross Unrealized Loss	22.00%
Fair Value Amount	57,144
Percent of Total Fair Value	14.00%
Due After Ten Years
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	856
Percent of Total Gross Unrealized Loss	7.00%
Fair Value Amount	30,532
Percent of Total Fair Value	8.00%
Commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	3,247
Percent of Total Gross Unrealized Loss	27.00%
Fair Value Amount	$ 105,323
Percent of Total Fair Value	26.00%