Business Combinations - Summary of Allocation of Purchase Consideration and Estimated Values of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Jan. 27, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets acquired
Goodwill		$ 31,328		$ 9,676	$ 7,839
IDV Solutions LLC
Assets acquired
Accounts receivable	$ 1,462
Other assets	242
Property and equipment	174
Goodwill	21,196		$ 21,200
Total assets acquired	30,794
Liabilities assumed
Accounts payable and accrued expenses	347
Deferred revenue	4,060
Other liabilities	132
Net assets acquired	26,255
Consideration paid
Cash paid, net of cash acquired	21,235
Acquisition date fair value of contingent consideration	5,020
Total	26,255
IDV Solutions LLC | Tradenames
Assets acquired
Intangible assets other than goodwill	1,590
IDV Solutions LLC | Acquired Technology
Assets acquired
Intangible assets other than goodwill	2,490
IDV Solutions LLC | Customer Relationships
Assets acquired
Intangible assets other than goodwill	3,400
IDV Solutions LLC | Non-compete Arrangement
Assets acquired
Intangible assets other than goodwill	$ 240