Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Cash and Cash Equivalents [Line Items]
Cash	$ 2,384,999	$ 2,572,685
Cash and cash equivalents	3,348,557	3,794,483
Non-current Assets, Cash	20,234	16,260
Restricted Cash and Cash Equivalents	21,540	17,558
Cash and Restricted Cash	2,405,233	2,588,945
Total Cash, cash equivalents, and restricted cash and cash equivalents	3,370,097	3,812,041
Fair Value, Measurements, Recurring | Level 1 Securities | Money market funds
Cash and Cash Equivalents [Line Items]
Money market funds	963,558	1,221,798
Money market funds | Fair Value, Measurements, Recurring | Level 1 Securities
Cash and Cash Equivalents [Line Items]
Non-current Assets, Securities	1,306	1,298
Cash Equivalents and Restricted Cash Equivalents	$ 964,864	$ 1,223,096