Tangible Equity Units - Proceeds from Tangible Equity Units Table (Detail) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) $ / shares $ / unit	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Tangible Equity Units [Line Items]
Net proceeds, debt component	$ 4,632	$ 2,879	$ 958
Tangible Equity Units - senior amortizing notes [Member] | Senior Notes [Member]
Tangible Equity Units [Line Items]
Price per TEU, debt component | $ / unit	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	$ 348	$ 0	$ 0
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