Accumulated Other Comprehensive Income - Accumulated Other Comprehensive Loss Balances (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ 16,593	$ 15,812	$ 14,885
Ending balance	17,386	16,593	15,812
Net Unrealized Gains (Losses) on Cash Flow Hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	38	26	3
Other comprehensive income before reclassifications	(16)	111	78
Amounts reclassified from AOCI	(99)	(99)	(55)
Ending balance	(77)	38	26
Net Unrealized Gains (Losses) on Securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(152)	(146)	(329)
Other comprehensive income before reclassifications	(14)	6	194
Amounts reclassified from AOCI		(12)	(11)
Ending balance	(208)	(152)	(146)
Foreign Currency Translation Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(24)	(10)	(5)
Other comprehensive income before reclassifications		(14)	(5)
Amounts reclassified from AOCI	0	0	0
Ending balance	(22)	(24)	(10)
Pension and Other Benefits Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(612)	(599)	(314)
Other comprehensive income before reclassifications	(12)	(75)	0
Amounts reclassified from AOCI	35	62	(285)
Ending balance	(589)	(612)	(599)
Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(750)	(729)	(645)
Other comprehensive income before reclassifications	(40)	28	267
Amounts reclassified from AOCI	(106)	(49)	(351)
Ending balance	$ (896)	$ (750)	$ (729)