Common Stockholders' Equity and Preferred Securities - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Stockholders Equity [Line Items]
Number of shares repurchased	28,000,000
Repurchases of common stock	$ 1,844	$ 2,607	$ 1,232
Share repurchase program additional amount authorized	3,000
Amount remaining under repurchase programs	$ 3,000
Treasury stock, shares, retired	25,000,000
Treasury stock retired, average cost per share	$ 63
Common stock reserved for future issuance	10,000,000
Junior preferred stock outstanding	2
Junior preferred stock par value per share	$ 0.01
Preferred stock authorized to issue	250,000,000