Notes Payable - Components of Notes Payable (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Notes payable	$ 425	$ 425
Deferred loan costs	(2)	(2)
Less: current portion	(21)	(40)
Notes payable, noncurrent	402	383
Notes Payable | Tranche A Term Loans
Line of Credit Facility [Line Items]
Notes payable	0	303
Notes Payable | Tranche A-2 Term Loans
Line of Credit Facility [Line Items]
Notes payable	0	122
Notes Payable | 2018 Term Loan A
Line of Credit Facility [Line Items]
Notes payable	$ 425	$ 0
Annual contractual interest rate	3.78%
Effective interest rate	3.88%
Notes Payable | 2018 Term Loan A | LIBOR
Line of Credit Facility [Line Items]
Spread on variable rate	1.625%
Notes Payable | 2018 Term Loan A | Prime lending rate
Line of Credit Facility [Line Items]
Spread on variable rate	0.625%