Equity - Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions		3 Months Ended				9 Months Ended		12 Months Ended
		Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2015	Jun. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income Loss [Line Items]
Unrealized gain (loss) during period on derivatives qualifying as cash flow hedges, net of income tax expense (benefit)		$ 8		$ 16		$ (13)	$ 17
Reclassification to net income of losses (gains) on cash flow hedges, net of income tax expense (benefit)		(60)	$ (69)	(41)	$ (58)	(60)	(41)	$ (23)	$ (63)
Accumulated Other Comprehensive Income (Loss)		(84)	(94)	(79)	(96)	(84)	(79)	(52)	(99)
Gain (loss) on defined benefit plans, net of income taxes		1				5	(2)
Reclassification to net income of (gains) losses on cash flow hedges, net of income taxes	[1]	1		1		(24)	5
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax		9		17		(37)	22
Other comprehensive (loss) income, net of tax		10		17		(32)	20
Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Reclassification to net income of losses (gains) on cash flow hedges, net of income tax expense (benefit)	[1]	(78)	(79)	(56)	(57)	(78)	(56)	(54)	(61)
Designated as Hedging Instrument [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Unrealized gain (loss) during period on derivatives qualifying as cash flow hedges, net of income tax expense (benefit)		18	10	15	(1)	18	15	31	(2)
Qualified Pension Plan [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Gain (loss) on defined benefit plans		(16)	(17)	(27)	(27)	(16)	(27)	(21)	(25)
Gain (loss) on defined benefit plans, net of income taxes		1				5	(2)
Non-Qualified Pension Plans [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Gain (loss) on defined benefit plans		$ (8)	$ (8)	$ (11)	$ (11)	$ (8)	$ (11)	$ (8)	$ (11)

[1] Reclassification is reflected in the operating expenses line item in the condensed consolidated statements of income.