Amount and Percentage of Fixed Maturities by Rating (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Investment Securities [Line Items]
Rating	AA
Fair Value	$ 2,310,195	$ 2,047,873
Percent of Total	100.00%
Extremely strong
Investment Securities [Line Items]
Rating	AAA
Fair Value	446,347
Percent of Total	19.00%
Very strong
Investment Securities [Line Items]
Rating	AA
Fair Value	1,067,305
Percent of Total	46.00%
Strong
Investment Securities [Line Items]
Rating	A
Fair Value	623,183
Percent of Total	27.00%
Adequate
Investment Securities [Line Items]
Rating	BBB
Fair Value	159,354
Percent of Total	7.00%
Speculative
Investment Securities [Line Items]
Rating	BB & Below
Fair Value	13,534
Percent of Total	1.00%
Not rated
Investment Securities [Line Items]
Rating	NR
Fair Value	$ 472
Percent of Total	0.00%