Additional Cash Flow Information - Condensed Consolidated Statements of Cash Flows (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 2,893,885	$ 2,650,134	$ 1,995,893	$ 1,665,412
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	2,901,532	2,658,253	2,005,728	1,667,526
Prepaid expenses and other current assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted Cash and Cash Equivalents	6,250	4,910	9,835	2,114
Other assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted Cash and Cash Equivalents	$ 1,397	$ 3,209	$ 0	$ 0