Significant Accounting Policies (Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 11,836	$ 10,717	$ 9,990
Other comprehensive income (loss)	11	(3)	5
Implementation of ASU 2018-02		0
Ending balance	12,589	11,836	10,717
AOCI including portion attributable to noncontrolling interest
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(148)	(120)
Other comprehensive income (loss) before reclassifications	(4)	(20)
Amounts reclassified from Accumulated other comprehensive income (loss)	15	17
Other comprehensive income (loss)	11	(3)
Implementation of ASU 2018-02		(25)
Ending balance	(137)	(148)	(120)
Net Unrealized Gain (Loss) on Derivatives
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(11)
Ending balance			(11)
Net Unrealized Gain (Loss) on Derivatives
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(25)
Other comprehensive income (loss) before reclassifications	(3)	(14)
Amounts reclassified from Accumulated other comprehensive income (loss)	5	2
Other comprehensive income (loss)	2	(12)
Implementation of ASU 2018-02		(2)
Ending balance	(23)	(25)
Benefit Obligations
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(117)	(102)
Other comprehensive income (loss) before reclassifications	(2)	(7)
Amounts reclassified from Accumulated other comprehensive income (loss)	10	15
Other comprehensive income (loss)	8	8
Implementation of ASU 2018-02		(23)
Ending balance	(109)	(117)	(102)
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(6)	(7)
Other comprehensive income (loss) before reclassifications	1	1
Amounts reclassified from Accumulated other comprehensive income (loss)	0	0
Other comprehensive income (loss)	1	1
Implementation of ASU 2018-02		0
Ending balance	$ (5)	$ (6)	$ (7)