Nature of Business and Accounting Policies - Schedule of Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 2,650,134	$ 1,665,412	$ 1,183,945	$ 714,768
Cash, cash equivalents and restricted cash per statement of cash flows	2,658,253	1,667,526	1,231,707	815,761
Prepaid and other current assets
Cash and Cash Equivalents [Line Items]
Restricted cash and cash equivalents	4,910	2,114	47,762	78,910
Other assets
Cash and Cash Equivalents [Line Items]
Restricted cash and cash equivalents	$ 3,209	$ 0	$ 0	$ 22,083