Basis of Presentation and Accounting Policies - Schedule of cash, cash equivalents and restricted cash (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 2,145,359	$ 1,665,412	$ 1,223,130	$ 1,183,945
Restricted cash and cash equivalents (VIE)	8,510	1,489	64,628	47,762
Cash, cash equivalents and restricted cash per statement of cash flows	2,154,068	1,667,526	1,287,758	1,231,707
Prepaid and other current assets
Cash and Cash Equivalents [Line Items]
Restricted cash and cash equivalents (VIE)	$ 199	$ 625	$ 0	$ 0