Investments - Credit Ratings of Fixed Maturity Securities and Short-Term Investments Classified as Trading (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	$ 2,742,646	$ 2,445,638
% of Total Fair Value	100.00%	100.00%
AAA [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	404,395	881,951
% of Total Fair Value	14.70%	36.00%
AA [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	1,057,034	400,394
% of Total Fair Value	38.50%	16.40%
A [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	893,069	796,608
% of Total Fair Value	32.60%	32.60%
BBB or lower [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	370,240	341,307
% of Total Fair Value	13.50%	14.00%
Not Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	$ 17,908	$ 25,378
% of Total Fair Value	0.70%	1.00%