Summary of Significant Accounting Policies - Adjustments Made Due to ASU 2016-02 (Details) - USD ($) $ in Millions	Sep. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
Assets
Right-of-use assets, operating leases	$ 99	$ 75
Current liabilities
Other current liabilities	18	16
Non-current liabilities
Operating lease non-current liabilities	81	57
Equity
Accumulated deficit	$ (584)	(181)	$ (181)
Accounting Standards Update 2016-02
Assets
Right-of-use assets, operating leases		75
Current liabilities
Other current liabilities		16
Non-current liabilities
Operating lease non-current liabilities		57
Equity
Accumulated deficit		$ 0