Tangible Equity Units - Proceeds from Tangible Equity Units Table (Detail) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) $ / unit	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) $ / shares $ / unit	Sep. 30, 2016 $ / shares
Tangible Equity Units [Line Items]
Price per TEU, equity component | $ / shares				$ 53.23
Net proceeds, debt component	$ 369	$ 6,042	$ 4,632
TEUs - senior amortizing notes due 2018 [Member]
Tangible Equity Units [Line Items]
Price per TEU, debt component | $ / unit	10.95		10.95
Gross proceeds, debt component			$ 101
Issuance costs, debt component			4
Net proceeds, debt component			$ 97
Equity Component of 7.5% Tangible Equity Units [Member]
Tangible Equity Units [Line Items]
Price per TEU, equity component | $ / shares			$ 39.05
Gross proceeds, equity component			$ 359
Issuance costs, equity component			11
Net proceeds, equity component	$ 0	$ 0	$ 348
7.50% Tangible Equity Units [Member]
Tangible Equity Units [Line Items]
Total price per TEU | $ / unit			50.00
Total gross proceeds			$ 460
Total issuance costs			15
Total net proceeds			$ 445