Amount and Percentage of Fixed Maturities by Rating (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Investment Securities [Line Items]
Rating	AA
Fair Value	$ 2,007,806	$ 2,121,833
Percent of Total	100.00%
Extremely strong
Investment Securities [Line Items]
Rating	AAA
Fair Value	293,952
Percent of Total	15.00%
Very strong
Investment Securities [Line Items]
Rating	AA
Fair Value	1,088,098
Percent of Total	53.00%
Strong
Investment Securities [Line Items]
Rating	A
Fair Value	454,692
Percent of Total	23.00%
Adequate
Investment Securities [Line Items]
Rating	BBB
Fair Value	152,317
Percent of Total	8.00%
Speculative
Investment Securities [Line Items]
Rating	BB & Below
Fair Value	15,073
Percent of Total	1.00%
Not rated
Investment Securities [Line Items]
Rating	NR
Fair Value	$ 3,674
Percent of Total	0.00%