Investments - Credit Ratings of Fixed Maturity and Short-Term Investments, Trading (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	$ 3,619,174	$ 3,662,721
Percentage of Total Fair Value	100.00%	100.00%
AAA [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	517,665	502,057
Percentage of Total Fair Value	14.30%	13.70%
AA [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	1,417,658	1,430,107
Percentage of Total Fair Value	39.10%	39.10%
A [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	1,156,852	1,191,142
Percentage of Total Fair Value	32.00%	32.50%
BBB or Lower [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	505,664	461,614
Percentage of Total Fair Value	14.00%	12.60%
Not Rated [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	$ 21,335	$ 77,801
Percentage of Total Fair Value	0.60%	2.10%