Equity (Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Accumulated other comprehensive loss, beginning	$ (70)	$ (167)	$ (40)
Other comprehensive loss before reclassifications	197	45	(164)
Amounts reclassified from AOCI	(4)	52	26
Total other comprehensive income (loss), net of tax	193	97	(138)
Less other comprehensive income attributable to noncontrolling interests	12	0	(11)
Accumulated other comprehensive loss, ending	111	(70)	(167)
Net Unrealized Gains (Losses) on Cash Flow Hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Accumulated other comprehensive loss, beginning	(100)	(170)	(156)
Other comprehensive loss before reclassifications	0	0	(88)
Amounts reclassified from AOCI	32	70	63
Total other comprehensive income (loss), net of tax	32	70	(25)
Less other comprehensive income attributable to noncontrolling interests	9	0	(11)
Accumulated other comprehensive loss, ending	(77)	(100)	(170)
Net Unrealized Gains (Losses) on Available for Sale Securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Accumulated other comprehensive loss, beginning	225	174	218
Other comprehensive loss before reclassifications	127	69	(7)
Amounts reclassified from AOCI	(36)	(18)	(37)
Total other comprehensive income (loss), net of tax	91	51	(44)
Less other comprehensive income attributable to noncontrolling interests	0	0	0
Accumulated other comprehensive loss, ending	316	225	174
Defined Benefit Pension and Other Benefits Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Accumulated other comprehensive loss, beginning	(83)	(62)	(20)
Other comprehensive loss before reclassifications	44	(21)	(42)
Amounts reclassified from AOCI	0	0	0
Total other comprehensive income (loss), net of tax	44	(21)	(42)
Less other comprehensive income attributable to noncontrolling interests	0	0	0
Accumulated other comprehensive loss, ending	(39)	(83)	(62)
Net Unrealized Gains (Losses) on Foreign Currency Translation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Accumulated other comprehensive loss, beginning	(90)	(85)	(58)
Other comprehensive loss before reclassifications	24	(5)	(27)
Amounts reclassified from AOCI	0	0	0
Total other comprehensive income (loss), net of tax	24	(5)	(27)
Less other comprehensive income attributable to noncontrolling interests	3	0	0
Accumulated other comprehensive loss, ending	(69)	(90)	(85)
Other Comprehensive Income (Loss) Related to Equity Method Investee
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Accumulated other comprehensive loss, beginning	(22)	(24)	(24)
Other comprehensive loss before reclassifications	2	2	0
Amounts reclassified from AOCI	0	0	0
Total other comprehensive income (loss), net of tax	2	2	0
Less other comprehensive income attributable to noncontrolling interests	0	0	0
Accumulated other comprehensive loss, ending	$ (20)	$ (22)	$ (24)