Amount and Percentage of Fixed Maturities by Rating (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Investment Securities [Line Items]
Rating	AA
Fair Value	$ 2,162,713	$ 2,047,873
Percent of Total	100.00%
Extremely strong
Investment Securities [Line Items]
Rating	AAA
Fair Value	389,209
Percent of Total	18.00%
Very strong
Investment Securities [Line Items]
Rating	AA
Fair Value	1,014,656
Percent of Total	47.00%
Strong
Investment Securities [Line Items]
Rating	A
Fair Value	561,731
Percent of Total	26.00%
Adequate
Investment Securities [Line Items]
Rating	BBB
Fair Value	175,311
Percent of Total	8.00%
Speculative
Investment Securities [Line Items]
Rating	BB & Below
Fair Value	18,287
Percent of Total	1.00%
Not rated
Investment Securities [Line Items]
Rating	NR
Fair Value	$ 3,519
Percent of Total	0.00%