Fair Value Measurements - Narrative (Details) $ / shares in Units, shares in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) interest_rate_swap_agreement $ / shares shares	Dec. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($)	Oct. 06, 2017 USD ($) $ / shares
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	$ 8,542,000,000	$ 8,965,000,000
Change in fair value of derivative liability	620,000,000	(697,000,000)	$ (64,000,000)
Derivative, notional amount	3,000,000,000.0
Accumulated other comprehensive income/(loss) before tax	(53,000,000)	(16,000,000)
AOCI reclassified to Interest expense	10,000,000	$ 0
Estimated amount of existing losses that are reported in AOCI at the reporting date that are expected to be reclassified into earnings within the next 12 months	$ 29,000,000
Convertible Debt | Fair Value, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible number of shares (in shares) | shares	156
Conversion price (in dollars per share) | $ / shares	$ 192.55	$ 122.38
Stated rate	4.00%	7.00%
Convertible Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Conversion price (in dollars per share) | $ / shares				$ 7.19
Convertible Debt | Unsecured Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt			$ 1,100,000,000
Face amount	$ 1,100,000,000			$ 1,100,000,000
Designated as Hedging Instrument
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of interest rate derivatives held | interest_rate_swap_agreement	10
Liability | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Changes in fair value related to the disputed claims liability	$ 20,000,000	$ 24,000,000
Measurement Input, Discount Rate | Convertible Debt | Fair Value, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate principal amount	5.00%
Measurement Input, Expected Term | Convertible Debt | Fair Value, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt term	7 years
Parent Company | Convertible Debt | Fair Value, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible number of shares (in shares) | shares	151