Common Stockholders' Equity and Preferred Securities - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule Of Stockholders Equity [Line Items]
Number of shares repurchased	50,000,000
Repurchases of common stock	$ 2,607	$ 1,232	$ 320
Amount remaining under repurchase programs	$ 1,800
Treasury stock, shares, retired	70,000,000
Treasury stock retired, average cost per share	$ 53
Common stock reserved for future issuance	5,000,000
Junior preferred stock outstanding	2
Junior preferred stock par value per share	$ 0.01
Preferred stock authorized to issue	250,000,000