Revenue Recognition - Accounts Receivable and Deferred Revenue (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Assets
Accounts receivable, balance	$ 44,089
Accounts receivable, additions	55,004
Accounts receivable, deductions	(48,767)
Accounts receivable, balance	50,326
Liabilities:
Deferred revenue, current portion, balance	1,420
Deferred revenue, current portion, additions	964
Deferred revenue, current portion, deductions	(941)
Deferred revenue, current portion, balance	1,443
Deferred revenue, noncurrent, balance
Deferred revenue, noncurrent, additions	35,462
Deferred revenue, noncurrent, balance	$ 35,462