Prepaid Expenses and Other Current Assets, Accrued Expenses, Other Current Liabilities, and Other Liabilities - Components of Other Current Liabilities and Other Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Other current liabilities
Sales tax payable	$ 49,989	$ 35,726
Sales return reserve	35,707	21,243
Other current liabilities	42,299	28,057
Other current liabilities	127,995	85,026
Other liabilities:
Deferred rent	99,746	59,811
Construction costs under build-to-suit leases	60,089	37,545
Other liabilities	553	9,136
Total other liabilities	$ 160,388	$ 106,492