Summary of Significant Accounting Policies (Accumulated Other Comprehensive Income) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), Pension and PRB Plans, Net - Beginning balance	$ (8,483,000,000)	$ (7,843,000,000)	$ (7,234,000,000)
Pension and PRB plans, net - Before tax amount	(822,000,000)	1,028,000,000	(651,000,000)
Pension and PRB plans, net - Tax (expense) benefit	173,000,000	(216,000,000)	42,000,000
Pension and PRB plans, net - Net of tax amount	(649,000,000)	812,000,000	(609,000,000)
Pension and PRB plans, net, Tax Cuts and Jobs Act of 2017, Reclassification from AOCI to Retained Earnings		(1,452,000,000)
Accumulated Other Comprehensive Income (Loss), Pension and PRB Plans, Net - Ending balance	(9,132,000,000)	(8,483,000,000)	(7,843,000,000)
Accumulated Other Comprehensive Income (Loss), Foreign Exchange Translation - Beginning balance	(131,000,000)	(95,000,000)	(175,000,000)
Foreign exchange translation - Before tax amount	7,000,000	(36,000,000)	80,000,000
Foreign exchange translation - Tax (expense) benefit	0	0	0
Foreign exchange translation - Net of tax amount	7,000,000	(36,000,000)	80,000,000
Foreign exchange translation, Tax Cuts and Jobs Act of 2017, Reclassification from AOCI to Retained Earnings		0
Accumulated Other Comprehensive Income (Loss), Foreign Exchange Translation - Ending balance	(124,000,000)	(131,000,000)	(95,000,000)
Accumulated Other Comprehensive Income (Loss), Cash flow hedges - beginning balance	(2,000,000)	6,000,000	0
Cash flow hedges - Before tax amount	4,000,000	(12,000,000)	10,000,000
Cash flow hedges - Tax (expense) benefit	(1,000,000)	3,000,000	(4,000,000)
Cash flow hedges - Net of tax amount	3,000,000	(9,000,000)	6,000,000
Cash flow hedges, Tax Cuts and Jobs Act of 2017, Reclassification from AOCI to Retained Earnings		1,000,000
Accumulated Other Comprehensive Income (Loss), Cash flow hedges - ending balance	1,000,000	(2,000,000)	6,000,000
Accumulated Other Comprehensive Income (Loss), Unrealized gains (losses) on investments, and other, net - Beginning balance	(2,000,000)	(3,000,000)	(2,000,000)
Unrealized gains (losses) on investments and other, net - Before tax amount	(3,000,000)	1,000,000	(1,000,000)
Unrealized gains (losses) on investments and other, net - tax (expense) benefit	0	0	0
Unrealized gains (losses) on investments and other, net, net of tax amount	(3,000,000)	1,000,000	(1,000,000)
Unrealized gains (losses) on investments and other, net, Tax Cuts and Jobs Act of 2017, Reclassification from AOCI to Retained Earnings		0
Accumulated Other Comprehensive Income (Loss), Unrealized gains (losses) on investments, and other, net - Ending balance	(5,000,000)	(2,000,000)	(3,000,000)
Accumulated other comprehensive income (loss), net of tax, - Beginning balance	(8,618,000,000)	(7,935,000,000)	(7,411,000,000)
Other comprehensive income (loss) - Before tax amount	(814,000,000)	981,000,000	(562,000,000)
Other comprehensive income (loss) - Tax (expense) benefit	172,000,000	(213,000,000)	38,000,000
Other comprehensive income (loss), net of tax	(642,000,000)	768,000,000	(524,000,000)
Reclassification of stranded tax effects	0	(1,451,000,000)	0
Accumulated other comprehensive income (loss) net of tax, - Ending balance	(9,260,000,000)	(8,618,000,000)	$ (7,935,000,000)
Pension and PRB plans, net - cumulative tax benefit	2,428,000,000	2,255,000,000
Cash flow hedges - cumulative tax benefit (expense)	0	1,000,000
Unrealized gains (losses) on investments and other, net - cumulative tax benefit (expense)	$ (1,000,000)	$ (1,000,000)