Shareholders' Equity and Dividend Restrictions - Additional Information (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Stockholders Equity Note [Line Items]
Dividends declared by insurance subsidiary to parent	$ 180	$ 220	$ 50
Dividend payment without prior regulatory approval, Minimum percent of policyholder surplus	10.00%
Dividend payment without prior regulatory approval, percent of statutory net income for the prior calendar year	100.00%
Dividends might be paid during 2012	$ 375
Declared cash dividends, per share	$ 1.605	$ 1.590	$ 1.570
Shares of common stock, available for future equity award grants	4.4