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Debt - Senior Notes (Details) - USD ($)	9 Months Ended	12 Months Ended
Debt - Senior Notes (Details) - USD ($)	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Aug. 19, 2019	May 23, 2019	Jan. 15, 2019
Debt Instrument [Line Items]
Proceeds from Debt, Net of Issuance Costs		$ 1,380,000,000
Debt Instrument, Redemption Price, Percentage of Principal Amount Redeemed		100.00%
Loss on extinguishment of debt		$ (90,000,000)	$ (6,000,000)	$ 0
Senior Note Five Point Seven Five Percent Due in Two Thousand Twenty Three [Member]
Debt Instrument [Line Items]
Debt repaid		265,000,000
Senior Note Two Point Eight due in Two Thousand Twenty Nine [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Increase (Decrease)	2000.00%
Senior Note Three Point Six due in Two Thousand Forty Nine [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Increase (Decrease)	2500.00%
Senior Note Five Point One Twenty Five due in Two Thousand Twenty Two [Member]
Debt Instrument [Line Items]
Debt repaid		$ 800
Senior Notes \| Senior Note Six Point Zero Percent Due in Two Thousand Nineteen [Member]
Debt Instrument [Line Items]
Debt repaid							$ 413,000,000
Senior Notes \| Senior Note Two Point Eight due in Two Thousand Twenty Nine [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount					$ 600,000,000
Senior Notes \| Senior Note Three Point Six due in Two Thousand Forty Nine [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount					$ 800,000,000
Par Value [Member] \| Senior Note Five Point Seven Five Percent Due in Two Thousand Twenty Three [Member]
Debt Instrument [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities						$ 265,000,000
Fair Value [Member] \| Senior Note Five Point Seven Five Percent Due in Two Thousand Twenty Three [Member]
Debt Instrument [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities						$ 284,000,000