Significant Accounting Policies - Reconciliation of cash, cash equivalents and restricted cash (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Cash and cash equivalents	$ 2,443	$ 2,847
Restricted cash and cash equivalents	259	69
Restricted cash included within long-term investments and other assets	3	1
Total cash, cash equivalents and restricted cash and cash equivalents in the consolidated statement of cash flow	$ 2,705	$ 2,917	$ 1,818	$ 1,713