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	Jun. 30, 2011	Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	$ 8,288	$ 20,762
Percent of Total Gross Unrealized Loss	100.00%
Fair Value Amount	424,288	757,305
Percent of Total Fair Value	100.00%
Due in one year or less
Schedule of Available-for-sale Securities [Line Items]
Fair Value Amount	7
Due after one year through five years
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	730
Percent of Total Gross Unrealized Loss	9.00%
Fair Value Amount	93,496
Percent of Total Fair Value	22.00%
Due after five years through ten years
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	2,258
Percent of Total Gross Unrealized Loss	27.00%
Fair Value Amount	109,933
Percent of Total Fair Value	26.00%
Due After Ten Years
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	1,293
Percent of Total Gross Unrealized Loss	16.00%
Fair Value Amount	42,316
Percent of Total Fair Value	10.00%
Commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss Amount	4,007
Percent of Total Gross Unrealized Loss	48.00%
Fair Value Amount	$ 178,536
Percent of Total Fair Value	42.00%