Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Thousands			3 Months Ended								5 Months Ended	9 Months Ended
	Mar. 13, 2015	Nov. 10, 2014	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Mar. 13, 2015	Sep. 30, 2015	Sep. 30, 2014	Jul. 03, 2015	Aug. 31, 2014	Apr. 30, 2014	Apr. 30, 2013
Equity, Class of Treasury Stock [Line Items]
Cash dividend per share			$ 0.29	$ 0.29	$ 0.28	$ 0.28	$ 0.28	$ 0.28	$ 0.27	$ 0.27
Share repurchase authorization															$ 1,000,000,000
Remaining authorized amount															$ 816,000,000
Value of shares repurchased												$ 329,000,000	$ 230,000,000
Shares repurchased in open market												1,850	1,870
Common shares acquired in connection with employee stock plans, shares												300	400
Common shares acquired in connection with employee stock plans, amount												$ 51,000,000	$ 40,000,000
Treasury stock reissued (shares)												700
Treasury stock reissued												$ 44,000,000
Accumulated other comprehensive income			$ 137,000,000			$ 223,000,000						137,000,000
Net unrealized gains on our investment securities
Equity, Class of Treasury Stock [Line Items]
Accumulated other comprehensive income			142,000,000			301,000,000						142,000,000
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
Equity, Class of Treasury Stock [Line Items]
Accumulated other comprehensive income			$ 5,000,000			$ 78,000,000						$ 5,000,000
September 2014 Share Repurchase Authorization
Equity, Class of Treasury Stock [Line Items]
Share repurchase authorization							$ 2,000,000,000						2,000,000,000
Remaining authorized amount														$ 1,040,000,000
Expiration date of new authorization												Dec. 31, 2016
Value of shares repurchased												$ 329,000,000	$ 35,000,000
Shares repurchased in open market												1,850	270
Accelerated Share Repurchase Agreement
Equity, Class of Treasury Stock [Line Items]
Share repurchase authorization		$ 500,000,000
Value of shares repurchased		$ 400,000,000									$ 500,000,000
Shares repurchased in open market		3,060									3,420
Decrease in capital in excess of par value		$ 100,000,000
ASR, shares received	360
Shares repurchased in open market, average price											$ 146.21
April 2014 Share Repurchase Authorization
Equity, Class of Treasury Stock [Line Items]
Share repurchase authorization																$ 1,000,000,000
Value of shares repurchased												$ 0	$ 184,000,000
Shares repurchased in open market												0	1,500
April 2013 Share Repurchase Authorization
Equity, Class of Treasury Stock [Line Items]
Share repurchase authorization																	$ 1,000,000,000
Value of shares repurchased												$ 0	$ 11,000,000
Shares repurchased in open market												0	100