Schedule II, Valuation and Qualifying Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Expense Related to Revaluation of Federal Benefit - TCJA	$ 14
Reduced Expense Related to Revaluation of Federal Benefits - TCJA	4
Allowance for Bad Debts
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Jan. 1	52	$ 51	$ 52
Charged to costs and expenses	42	39	39
Charged to other accounts	11	10	11
Deductions from reserves	(50)	(48)	(51)
Balance at Dec. 31	55	52	51
NOL and Tax Credit Valuation Allowances
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Jan. 1	77	58	28
Charged to costs and expenses	7	9	3
Charged to other accounts	0	22	35
Deductions from reserves	(5)	(12)	(8)
Balance at Dec. 31	$ 79	$ 77	$ 58