Amount and Percentage of Fixed Maturities by Rating (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Investment Securities [Line Items]
Rating	AA
Fair Value	$ 1,981,731	$ 2,121,833
Percent of Total	100.00%
Extremely strong
Investment Securities [Line Items]
Rating	AAA
Fair Value	305,979
Percent of Total	15.00%
Very strong
Investment Securities [Line Items]
Rating	AA
Fair Value	1,051,156
Percent of Total	54.00%
Strong
Investment Securities [Line Items]
Rating	A
Fair Value	440,063
Percent of Total	22.00%
Adequate
Investment Securities [Line Items]
Rating	BBB
Fair Value	163,614
Percent of Total	8.00%
Speculative
Investment Securities [Line Items]
Rating	BB & Below
Fair Value	11,722
Percent of Total	1.00%
Not rated
Investment Securities [Line Items]
Rating	NR
Fair Value	$ 9,197
Percent of Total	0.00%