Revenue - Revenue by Category (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Revenue	$ 149,413	$ 133,691	$ 288,886	$ 258,723
Automotive Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	45,760	31,319	83,534	58,479
Financial and Insurance Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	22,170	26,293	43,291	51,476
Manufacturing Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	14,190	12,647	28,478	25,935
Energy, Oil and Gas Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	10,306	11,662	22,864	20,786
U.S. Government Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	11,851	8,831	23,375	17,671
UK Government Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	4,357	4,947	8,412	10,433
Information and Communication Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	6,075	6,661	11,840	12,335
Aerospace Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	7,974	7,679	15,587	16,162
Electronics Semiconductor Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	4,433	4,055	8,809	7,789
Life Sciences Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	6,620	5,504	13,447	10,062
Other Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	15,677	14,093	29,249	27,595
Managed Learning Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	52,253	53,080	104,071	104,857
Engineering and Technical Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	28,806	29,002	56,438	53,671
Sales Enablement [Member]
Disaggregation of Revenue [Line Items]
Revenue	44,764	27,799	81,928	51,649
Organizational Development [Member]
Disaggregation of Revenue [Line Items]
Revenue	23,590	23,810	46,449	48,546
Reportable Geographical Components [Member] | Americas [Member]
Disaggregation of Revenue [Line Items]
Revenue	110,773	98,684	211,440	188,305
Reportable Geographical Components [Member] | EMEA [Member]
Disaggregation of Revenue [Line Items]
Revenue	34,905	33,724	68,817	67,602
Reportable Geographical Components [Member] | Asia Pacific [Member]
Disaggregation of Revenue [Line Items]
Revenue	15,389	8,025	26,665	15,384
Geography Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue	(11,654)	(6,742)	(18,036)	(12,568)
Workforce Excellence [Member]
Disaggregation of Revenue [Line Items]
Revenue	81,059	82,082	160,509	158,528
Workforce Excellence [Member] | Automotive Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	2,129	2,962	3,822	5,876
Workforce Excellence [Member] | Financial and Insurance Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	19,770	23,042	38,397	45,158
Workforce Excellence [Member] | Manufacturing Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	8,416	8,887	16,394	18,063
Workforce Excellence [Member] | Energy, Oil and Gas Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	8,687	10,862	20,062	18,656
Workforce Excellence [Member] | U.S. Government Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	9,870	6,513	19,486	13,031
Workforce Excellence [Member] | UK Government Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	4,357	4,947	8,412	10,433
Workforce Excellence [Member] | Information and Communication Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	4,002	4,091	7,463	7,712
Workforce Excellence [Member] | Aerospace Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	7,102	7,110	13,554	14,914
Workforce Excellence [Member] | Electronics Semiconductor Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	4,093	3,826	8,215	7,509
Workforce Excellence [Member] | Life Sciences Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	4,996	2,977	9,708	4,851
Workforce Excellence [Member] | Other Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	7,637	6,865	14,996	12,325
Workforce Excellence [Member] | Managed Learning Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	52,253	53,080	104,071	104,857
Workforce Excellence [Member] | Engineering and Technical Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	28,806	29,002	56,438	53,671
Workforce Excellence [Member] | Sales Enablement [Member]
Disaggregation of Revenue [Line Items]
Revenue	0	0	0	0
Workforce Excellence [Member] | Organizational Development [Member]
Disaggregation of Revenue [Line Items]
Revenue	0	0	0	0
Workforce Excellence [Member] | Reportable Geographical Components [Member] | Americas [Member]
Disaggregation of Revenue [Line Items]
Revenue	57,799	54,171	112,484	102,871
Workforce Excellence [Member] | Reportable Geographical Components [Member] | EMEA [Member]
Disaggregation of Revenue [Line Items]
Revenue	22,468	24,466	44,697	49,100
Workforce Excellence [Member] | Reportable Geographical Components [Member] | Asia Pacific [Member]
Disaggregation of Revenue [Line Items]
Revenue	8,693	7,908	14,835	15,195
Workforce Excellence [Member] | Geography Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue	(7,901)	(4,463)	(11,507)	(8,638)
Business Transformation Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	68,354	51,609	128,377	100,195
Business Transformation Services [Member] | Automotive Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	43,631	28,357	79,712	52,603
Business Transformation Services [Member] | Financial and Insurance Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	2,400	3,251	4,894	6,318
Business Transformation Services [Member] | Manufacturing Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	5,774	3,760	12,084	7,872
Business Transformation Services [Member] | Energy, Oil and Gas Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,619	800	2,802	2,130
Business Transformation Services [Member] | U.S. Government Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,981	2,318	3,889	4,640
Business Transformation Services [Member] | UK Government Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	0	0	0	0
Business Transformation Services [Member] | Information and Communication Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	2,073	2,570	4,377	4,623
Business Transformation Services [Member] | Aerospace Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	872	569	2,033	1,248
Business Transformation Services [Member] | Electronics Semiconductor Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	340	229	594	280
Business Transformation Services [Member] | Life Sciences Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,624	2,527	3,739	5,211
Business Transformation Services [Member] | Other Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue	8,040	7,228	14,253	15,270
Business Transformation Services [Member] | Managed Learning Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	0	0	0	0
Business Transformation Services [Member] | Engineering and Technical Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	0	0	0	0
Business Transformation Services [Member] | Sales Enablement [Member]
Disaggregation of Revenue [Line Items]
Revenue	44,764	27,799	81,928	51,649
Business Transformation Services [Member] | Organizational Development [Member]
Disaggregation of Revenue [Line Items]
Revenue	23,590	23,810	46,449	48,546
Business Transformation Services [Member] | Reportable Geographical Components [Member] | Americas [Member]
Disaggregation of Revenue [Line Items]
Revenue	52,974	44,513	98,956	85,434
Business Transformation Services [Member] | Reportable Geographical Components [Member] | EMEA [Member]
Disaggregation of Revenue [Line Items]
Revenue	12,437	9,258	24,120	18,502
Business Transformation Services [Member] | Reportable Geographical Components [Member] | Asia Pacific [Member]
Disaggregation of Revenue [Line Items]
Revenue	6,696	117	11,830	189
Business Transformation Services [Member] | Geography Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue	$ (3,753)	$ (2,279)	$ (6,529)	$ (3,930)