Contractual Maturity Dates for Fixed-Maturity and Short-Term Investments (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Amortized cost
Less than 1 year	$ 394
Years 1 - 5	2,771
Years 6 - 10	3,614
Years 11 - 20	1,081
Over 20 years	224
Total	8,084	7,888
Fair value
Less than 1 year	401
Years 1 - 5	2,923
Years 6 - 10	4,028
Years 11 - 20	1,175
Over 20 years	252
Total	$ 8,779	$ 8,383
% of fair value
Less than 1 year	4.60%
Years 1 - 5	33.30%
Years 6 - 10	45.80%
Years 11 - 20	13.40%
Over 20 years	2.90%
Total	100.00%