Summary of Significant Accounting Policies - Commission Payments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounting for Commission Payments
Commissions deferred during the period	$ 33,300	$ 42,000	$ 31,700
Deferred commissions	36,298	35,910
Sales and marketing
Accounting for Commission Payments
Amortization of deferred commissions	$ 33,000	$ 32,300	$ 22,100
Minimum
Accounting for Commission Payments
Commission payment period	45 days
Maximum
Accounting for Commission Payments
Commission payment period	75 days