Additional Cash Flow Information - Condensed Consolidated Statements of Cash Flows (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 3,294,684	$ 2,650,134	$ 2,145,359	$ 1,665,412
Cash, cash equivalents and restricted cash per statement of cash flows	3,301,646	2,658,253	2,154,068	1,667,526
Prepaid expenses and other current assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash and cash equivalents	6,962	4,910	8,709	2,114
Other assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash and cash equivalents	$ 0	$ 3,209	$ 0	$ 0