Summary of Significant Accounting Policies - Commission Payments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting for Commission Payments
Commissions deferred during the period	$ 48,200	$ 33,300	$ 42,000
Deferred commissions	42,806	36,298
Sales and marketing
Accounting for Commission Payments
Amortization of deferred commissions	$ 41,700	$ 33,000	$ 32,300
Minimum
Accounting for Commission Payments
Commission payment period	45 days
Maximum
Accounting for Commission Payments
Commission payment period	75 days