Acquisitions (Details) - USD ($) $ in Thousands					1 Months Ended	3 Months Ended	12 Months Ended
	Aug. 31, 2017	Apr. 01, 2017	Feb. 01, 2017	Oct. 01, 2016	Jul. 31, 2017	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High						$ 11,500	$ 11,500
Purchase price allocation:
Goodwill						176,124	$ 176,124	$ 144,835	$ 127,772
Acquired finite-lived intangible assets, weighted average useful life							6 years 2 months
Contingent Consideration Liability, Payments							$ 0
Contingent Consideration Payments							0	$ 4,657	$ 2,244
YouTrain [Member]
Business Acquisition [Line Items]
Cash purchase price	$ 4,898
Fair value of contingent consideration	0
Total purchase price	5,078
Purchase price allocation:
Fixed assets	215
Goodwill	3,268
Total assets	5,703
Accrued expenses	348
Deferred revenue	28
Deferred tax liability	249
Total liabilities	625
Net assets acquired	5,078
YouTrain [Member] | Marketing-Related Intangible Assets [Member]
Purchase price allocation:
Intangible assets	0
YouTrain [Member] | Technology-Based Intangible Assets [Member]
Purchase price allocation:
Intangible assets	0
Maverick Solutions [Member]
Business Acquisition [Line Items]
Cash purchase price				$ 4,639
Number of Twelve-Month Earnings Target Measurement Periods				2 years
Fair value of contingent consideration				$ 5,166
Total purchase price				9,805
Purchase price allocation:
Fixed assets				63
Goodwill				8,111
Total assets				10,166
Accrued expenses				38
Deferred revenue				323
Total liabilities				361
Net assets acquired				9,805
Contingent Consideration Liability, Payments							0
Contingent Consideration Payments						4,100
Maverick Solutions [Member] | Customer-related intangible assets [Member]
Purchase price allocation:
Intangible assets				$ 1,219
Acquired finite-lived intangible assets, weighted average useful life				4 years
Maverick Solutions [Member] | Marketing-Related Intangible Assets [Member]
Purchase price allocation:
Intangible assets				$ 124
Acquired finite-lived intangible assets, weighted average useful life				2 years
Maverick Solutions [Member] | Technology-Based Intangible Assets [Member]
Purchase price allocation:
Intangible assets				$ 649
Acquired finite-lived intangible assets, weighted average useful life				3 years
CLS [Member] [Member]
Business Acquisition [Line Items]
Cash purchase price	436
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High	2,200
Fair value of contingent consideration	888
Total purchase price	1,324
Purchase price allocation:
Fixed assets	0
Goodwill	1,090
Total assets	1,343
Accrued expenses	19
Deferred revenue	0
Deferred tax liability	0
Total liabilities	19
Net assets acquired	1,324
Contingent Consideration Liability, Payments							0
CLS [Member] [Member] | Marketing-Related Intangible Assets [Member]
Purchase price allocation:
Intangible assets	0
CLS [Member] [Member] | Technology-Based Intangible Assets [Member]
Purchase price allocation:
Intangible assets	$ 0
Emantras [Member]
Business Acquisition [Line Items]
Cash purchase price		$ 3,191
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High		300
Fair value of contingent consideration		220
Total purchase price		3,411
Purchase price allocation:
Fixed assets		50
Goodwill		3,156
Total assets		4,024
Accrued expenses		558
Deferred revenue		55
Deferred tax liability		0
Total liabilities		613
Net assets acquired		3,411
Contingent Consideration Liability, Payments							0
Emantras [Member] | Marketing-Related Intangible Assets [Member]
Purchase price allocation:
Intangible assets		0
Emantras [Member] | Technology-Based Intangible Assets [Member]
Purchase price allocation:
Intangible assets		$ 0
McKinney Rogers [Member]
Business Acquisition [Line Items]
Cash purchase price			$ 3,259
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High			$ 18,000			$ 8,000	8,000
Number of Twelve-Month Earnings Target Measurement Periods			3 years
Business Combination, Contingent Consideration, Term of Earnings Targets			5 months
Fair value of contingent consideration			$ 4,505
Total purchase price			7,764
Purchase price allocation:
Fixed assets			0
Goodwill			5,196
Total assets			8,674
Accrued expenses			44
Deferred revenue			866
Deferred tax liability			0
Total liabilities			910
Net assets acquired			7,764
Contingent Consideration Liability, Payments							$ 0
McKinney Rogers [Member] | Marketing-Related Intangible Assets [Member]
Purchase price allocation:
Intangible assets			$ 121
Acquired finite-lived intangible assets, weighted average useful life			3 years
McKinney Rogers [Member] | Technology-Based Intangible Assets [Member]
Purchase price allocation:
Intangible assets			$ 2,704
Target Earnings Measurement Period One [Member] | McKinney Rogers [Member]
Business Acquisition [Line Items]
Business Combination, Contingent Consideration, Amount Contingent Upon Achievement of Certain Earnings Targets			6,000
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability					$ 1,000
Target Earnings Measurement Period Two [Member] | McKinney Rogers [Member]
Business Acquisition [Line Items]
Business Combination, Contingent Consideration, Amount Contingent Upon Achievement of Certain Earnings Targets			$ 12,000