Summary of Significant Accounting Policies - Schedule of Cash and Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash and cash equivalents	$ 2,665,673	$ 3,367,914	$ 4,006,593	$ 3,393,216
Restricted cash	120,194	155,323	88,946	105,519
Restricted cash, net of current portion	433,841	441,722	330,223	268,165
Total as presented in the consolidated statements of cash flows	$ 3,219,708	$ 3,964,959	$ 4,425,762	$ 3,766,900