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Revenue Recognition - Narrative (Details) - USD ($)	3 Months Ended	9 Months Ended
Revenue Recognition - Narrative (Details) - USD ($)	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cumulative effect of accounting change					$ 174,000,000
Bad debts expense		$ 67,000,000	$ 64,000,000
Capitalized contract cost
Capitalized contract cost related to commission fees	$ 133,000,000	133,000,000
Amortization of contract cost assets	27,000,000	81,000,000
Impairment of capitalized contract cost	$ 0	$ 0
Minimum
Capitalized contract cost
Capitalized contract cost, amortization period	13 months	13 months
Maximum
Capitalized contract cost
Capitalized contract cost, amortization period	30 months	30 months
Retained earnings
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cumulative effect of accounting change					$ 173,000,000
Revenue from contract with customer
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Bad debts expense	$ 26,000,000	$ 67,000,000
ASU 2014-09 \| Retained earnings
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cumulative effect of accounting change				$ 173,000,000