Share-based Compensation - Share-based Compensation Expense (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Restricted stock, After Income Taxes	$ 39	$ 19	$ 35
Stock options, After Income Taxes	1	8	21
Performance share units , After Income Taxes	12	6	5
Total, After Income Taxes	52	33	61
Restricted stock, Before Income Taxes	62	31	57
Stock options, Before Income Taxes	2	13	34
Performance share units, Before Income Taxes	19	10	8
Total, Before Income Taxes	$ 83	$ 54	$ 99