Significant Accounting Policies (Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019	Jan. 01, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 10,717	$ 9,990	$ 9,499
Net current-period Other comprehensive income (loss)	(3)	5	13
Ending balance	11,836	10,717	9,990
ASU 2016-01
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Implementation of ASU					$ 0
ASU 2018-02
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Implementation of ASU				$ 0
AOCI including portion attributable to noncontrolling interest
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(120)	(120)
Other comprehensive income (loss) before reclassifications	(20)	(5)
Amounts reclassified from Accumulated other comprehensive income (loss)	17	10
Net current-period Other comprehensive income (loss)	(3)	5
Ending balance	(148)	(120)	(120)
AOCI including portion attributable to noncontrolling interest | ASU 2016-01
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Implementation of ASU					(5)
AOCI including portion attributable to noncontrolling interest | ASU 2018-02
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Implementation of ASU				(25)
Net Unrealized Gain (Loss) on Derivatives
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(11)	(3)
Other comprehensive income (loss) before reclassifications	(14)	(2)
Amounts reclassified from Accumulated other comprehensive income (loss)	2	1
Net current-period Other comprehensive income (loss)	(12)	(1)
Ending balance	(25)	(11)	(3)
Net Unrealized Gain (Loss) on Derivatives | ASU 2016-01
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Implementation of ASU					(7)
Net Unrealized Gain (Loss) on Derivatives | ASU 2018-02
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Implementation of ASU				(2)
Net Unrealized Loss on Investments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	0	(2)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from Accumulated other comprehensive income (loss)	0	0
Net current-period Other comprehensive income (loss)	0	0
Ending balance	0	0	(2)
Net Unrealized Loss on Investments | ASU 2016-01
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Implementation of ASU					2
Net Unrealized Loss on Investments | ASU 2018-02
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Implementation of ASU				0
Benefit Obligations
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(102)	(110)
Other comprehensive income (loss) before reclassifications	(7)	(1)
Amounts reclassified from Accumulated other comprehensive income (loss)	15	9
Net current-period Other comprehensive income (loss)	8	8
Ending balance	(117)	(102)	(110)
Benefit Obligations | ASU 2016-01
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Implementation of ASU					0
Benefit Obligations | ASU 2018-02
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Implementation of ASU				(23)
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(7)	(5)
Other comprehensive income (loss) before reclassifications	1	(2)
Amounts reclassified from Accumulated other comprehensive income (loss)	0	0
Net current-period Other comprehensive income (loss)	1	(2)
Ending balance	$ (6)	$ (7)	$ (5)
Foreign Currency Translation | ASU 2016-01
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Implementation of ASU					$ 0
Foreign Currency Translation | ASU 2018-02
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Implementation of ASU				$ 0