Accumulated Other Comprehensive Loss - Tax Impact (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, tax	$ 0	$ (4)	$ 20
Amounts reclassified from accumulated other comprehensive (income) loss, tax	5	12	(14)
Available-for-Sale Securities. [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, tax	2	3	(1)
Amounts reclassified from accumulated other comprehensive (income) loss, tax	(1)	0	0
Foreign Currency Translation Adjustments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, tax	0	0	0
Amounts reclassified from accumulated other comprehensive (income) loss, tax	0	0	0
Post - Retirement Benefit Obligation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, tax	1	0	1
Amounts reclassified from accumulated other comprehensive (income) loss, tax	$ 0	$ 1	$ 0