Stockholders' Equity - Additional Information (Detail) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended							1 Months Ended	9 Months Ended			1 Months Ended	9 Months Ended	1 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Sep. 30, 2014 Net unrealized gains on our investment securities	Dec. 31, 2013 Net unrealized gains on our investment securities	Sep. 30, 2014 Additional liability that would exist on closed block of long-term care policies assuming unrealized gains on investments backing such products were realized and reinvested at current yields	Dec. 31, 2013 Additional liability that would exist on closed block of long-term care policies assuming unrealized gains on investments backing such products were realized and reinvested at current yields	Oct. 31, 2014 Subsequent Event	Sep. 30, 2014 Prior Authorization	Sep. 30, 2013 Prior Authorization	Apr. 30, 2014 Prior Authorization	Sep. 30, 2014 September 2014 Share Repurchase Authorization	Sep. 30, 2014 September 2014 Share Repurchase Authorization	Nov. 06, 2014 September 2014 Share Repurchase Authorization Subsequent Event	Nov. 07, 2014 September 2014 Share Repurchase Authorization Subsequent Event
Equity, Class of Treasury Stock [Line Items]
Dividends per common share (in dollars per share)	$ 0.28	$ 0.27	$ 0.83	$ 0.80						$ 0.28
Share repurchase authorization													$ 1,000,000,000	$ 2,000,000,000	$ 2,000,000,000
Remaining authorized amount													816,000,000				1,870,000,000
Expiration date of new authorization															Dec. 31, 2016
Shares repurchased in open market											1.60	3.78			0.27	0.77
Value of shares repurchased											195,000,000	301,000,000			35,000,000	100,000,000
Shares repurchased in open market, average price											$ 121.68	$ 79.79			$ 128.34	$ 130.50
Share repurchase authorization														In September 2014, the Board of Directors replaced a previous share repurchase authorization of up to $1 billion (of which $816 million remained unused) with a new authorization for repurchases of up to $2 billion of our common shares exclusive of shares repurchased in connection with employee stock plans, expiring on December 31, 2016.
Common shares acquired in connection with employee stock plans, shares			0.4	0.3
Common shares acquired in connection with employee stock plans, amount			40,000,000	24,000,000
Accumulated other comprehensive income	$ 233,000,000		$ 233,000,000		$ 158,000,000	$ 286,000,000	$ 158,000,000	$ 53,000,000	$ 0