Stockholders' Equity - Additional Information (Detail) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	6 Months Ended							1 Months Ended	6 Months Ended		1 Months Ended	6 Months Ended	1 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2013 Net unrealized gains on our investment securities	Dec. 31, 2012 Net unrealized gains on our investment securities	Jun. 30, 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	Dec. 31, 2012 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	Apr. 30, 2013 Prior Authorization	Jun. 30, 2013 Prior Authorization	Jun. 30, 2012 Prior Authorization	Apr. 30, 2012 Current Authorization	Jun. 30, 2013 Current Authorization	Jul. 31, 2013 Subsequent Event Current Authorization
Equity, Class of Treasury Stock [Line Items]
Expiration date of current authorization												Jun. 30, 2015
Share repurchase authorization								$ 1,000,000,000			$ 1,000,000,000
Share repurchase authorization								In April 2013, the Board of Directors replaced its previously approved share repurchase authorization of up to $1 billion (of which $557 million remained unused) with the current authorization for repurchases of up to $1 billion of our common shares exclusive of shares repurchased in connection with employee stock plans, expiring on June 30, 2015
Shares repurchased in open market									1.22	2.73		1.61
Value of shares repurchased									82,000,000	226,000,000		129,000,000
Shares repurchased in open market, average price									$ 67.59	$ 82.78		$ 80.06
Remaining authorized amount								557,000,000					871,000,000
Common shares acquired in connection with employee stock plans, shares	0.2	0.6
Common shares acquired in connection with employee stock plans, amount	20,000,000	52,000,000
Accumulated other comprehensive income	$ 209,000,000		$ 386,000,000	$ 217,000,000	$ 462,000,000	$ 8,000,000	$ 76,000,000