Revenue - Revenue by Category (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Revenue	$ 155,399	$ 583,290	$ 515,160	$ 509,208
Automotive Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		165,129	116,077	111,387
Financial and Insurance Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		93,149	100,116	103,057
Manufacturing Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		56,048	49,211	52,929
Energy, Oil and Gas Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		41,297	41,840	36,624
U.S. Government Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		47,396	38,366	34,729
UK Government Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		18,153	18,733	27,734
Information and Communication Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		22,207	23,593	28,613
Aerospace Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		35,265	29,672	28,691
Electronics Semiconductor Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		15,401	15,927	17,518
Life Sciences Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		25,930	23,759	17,797
Other Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		63,315	57,866	50,129
Managed Learning Services [Member]
Disaggregation of Revenue [Line Items]
Revenue		218,730	206,388	207,007
Engineering and Technical Services [Member]
Disaggregation of Revenue [Line Items]
Revenue		111,065	110,426	101,252
Sales Enablement [Member]
Disaggregation of Revenue [Line Items]
Revenue		161,295	103,740	101,196
Organizational Development [Member]
Disaggregation of Revenue [Line Items]
Revenue		92,200	94,606	99,753
Reportable Geographical Components [Member] | Americas [Member]
Disaggregation of Revenue [Line Items]
Revenue		423,365	379,745	373,680
Reportable Geographical Components [Member] | EMEA [Member]
Disaggregation of Revenue [Line Items]
Revenue		142,107	129,935	130,757
Reportable Geographical Components [Member] | Asia Pacific [Member]
Disaggregation of Revenue [Line Items]
Revenue		59,654	33,322	30,204
Geography Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue		(41,836)	(27,842)	(25,433)
Workforce Excellence [Member]
Disaggregation of Revenue [Line Items]
Revenue		329,795	316,814	308,259
Workforce Excellence [Member] | Automotive Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue				10,102
Workforce Excellence [Member] | Financial and Insurance Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue				86,718
Workforce Excellence [Member] | Manufacturing Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue				35,795
Workforce Excellence [Member] | Energy, Oil and Gas Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue				34,195
Workforce Excellence [Member] | U.S. Government Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue				25,254
Workforce Excellence [Member] | UK Government Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue				27,734
Workforce Excellence [Member] | Information and Communication Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue				18,123
Workforce Excellence [Member] | Aerospace Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue				22,142
Workforce Excellence [Member] | Electronics Semiconductor Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue				16,449
Workforce Excellence [Member] | Life Sciences Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue				8,420
Workforce Excellence [Member] | Other Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue				23,327
Workforce Excellence [Member] | Managed Learning Services [Member]
Disaggregation of Revenue [Line Items]
Revenue		218,730	206,388	207,007
Workforce Excellence [Member] | Engineering and Technical Services [Member]
Disaggregation of Revenue [Line Items]
Revenue		111,065	110,426	101,252
Workforce Excellence [Member] | Sales Enablement [Member]
Disaggregation of Revenue [Line Items]
Revenue		0	0	0
Workforce Excellence [Member] | Organizational Development [Member]
Disaggregation of Revenue [Line Items]
Revenue		0	0	0
Workforce Excellence [Member] | Reportable Geographical Components [Member] | Americas [Member]
Disaggregation of Revenue [Line Items]
Revenue		230,236	213,938	198,653
Workforce Excellence [Member] | Reportable Geographical Components [Member] | EMEA [Member]
Disaggregation of Revenue [Line Items]
Revenue		91,947	91,764	100,296
Workforce Excellence [Member] | Reportable Geographical Components [Member] | Asia Pacific [Member]
Disaggregation of Revenue [Line Items]
Revenue		34,300	30,688	29,828
Workforce Excellence [Member] | Geography Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue		(26,688)	(19,576)	(20,518)
Business Transformation Services [Member]
Disaggregation of Revenue [Line Items]
Revenue		253,495	198,346	200,949
Business Transformation Services [Member] | Automotive Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue				101,285
Business Transformation Services [Member] | Financial and Insurance Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue				16,339
Business Transformation Services [Member] | Manufacturing Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue				17,134
Business Transformation Services [Member] | Energy, Oil and Gas Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue				2,429
Business Transformation Services [Member] | U.S. Government Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue				9,475
Business Transformation Services [Member] | UK Government Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue				0
Business Transformation Services [Member] | Information and Communication Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue				10,490
Business Transformation Services [Member] | Aerospace Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue				6,549
Business Transformation Services [Member] | Electronics Semiconductor Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue				1,069
Business Transformation Services [Member] | Life Sciences Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue				9,377
Business Transformation Services [Member] | Other Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue				26,802
Business Transformation Services [Member] | Managed Learning Services [Member]
Disaggregation of Revenue [Line Items]
Revenue		0	0	0
Business Transformation Services [Member] | Engineering and Technical Services [Member]
Disaggregation of Revenue [Line Items]
Revenue		0	0	0
Business Transformation Services [Member] | Sales Enablement [Member]
Disaggregation of Revenue [Line Items]
Revenue		161,295	103,740	101,196
Business Transformation Services [Member] | Organizational Development [Member]
Disaggregation of Revenue [Line Items]
Revenue		92,200	94,606	99,753
Business Transformation Services [Member] | Reportable Geographical Components [Member] | Americas [Member]
Disaggregation of Revenue [Line Items]
Revenue		193,129	165,807	175,027
Business Transformation Services [Member] | Reportable Geographical Components [Member] | EMEA [Member]
Disaggregation of Revenue [Line Items]
Revenue		50,160	38,171	30,461
Business Transformation Services [Member] | Reportable Geographical Components [Member] | Asia Pacific [Member]
Disaggregation of Revenue [Line Items]
Revenue		25,354	2,634	376
Business Transformation Services [Member] | Geography Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue		(15,148)	(8,266)	$ (4,915)
Other Market Sector [Member] | Workforce Excellence [Member] | Other Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		34,723	29,744
Other Market Sector [Member] | Business Transformation Services [Member] | Other Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		28,592	28,122
Life Sciences Market Sector [Member] | Workforce Excellence [Member] | Life Sciences Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		19,560	15,009
Life Sciences Market Sector [Member] | Business Transformation Services [Member] | Life Sciences Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		6,370	8,750
Electronics Semiconductor Market Sector [Member] | Workforce Excellence [Member] | Electronics Semiconductor Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		13,906	15,070
Electronics Semiconductor Market Sector [Member] | Business Transformation Services [Member] | Electronics Semiconductor Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		1,495	857
Aerospace Market Sector [Member] | Workforce Excellence [Member] | Aerospace Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		27,511	25,989
Aerospace Market Sector [Member] | Business Transformation Services [Member] | Aerospace Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		7,754	3,683
Information and Communication Market Sector [Member] | Workforce Excellence [Member] | Information and Communication Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		14,294	14,083
Information and Communication Market Sector [Member] | Business Transformation Services [Member] | Information and Communication Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		7,913	9,510
UK Government Market Sector [Member] | Workforce Excellence [Member] | UK Government Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		18,153	18,733
UK Government Market Sector [Member] | Business Transformation Services [Member] | UK Government Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		0	0
U.S. Government Market Sector [Member] | Workforce Excellence [Member] | U.S. Government Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		39,432	29,584
U.S. Government Market Sector [Member] | Business Transformation Services [Member] | U.S. Government Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		7,964	8,782
Energy, Oil and Gas Market Sector [Member] | Workforce Excellence [Member] | Energy, Oil and Gas Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		35,604	37,088
Energy, Oil and Gas Market Sector [Member] | Business Transformation Services [Member] | Energy, Oil and Gas Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		5,693	4,752
Manufacturing Market Sector [Member] | Workforce Excellence [Member] | Manufacturing Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		34,154	33,055
Manufacturing Market Sector [Member] | Business Transformation Services [Member] | Manufacturing Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		21,894	16,156
Financial and Insurance Market Sector [Member] | Workforce Excellence [Member] | Financial and Insurance Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		82,434	87,813
Financial and Insurance Market Sector [Member] | Business Transformation Services [Member] | Financial and Insurance Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		10,715	12,303
Automotive Market Sector [Member] | Workforce Excellence [Member] | Automotive Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		10,024	10,646
Automotive Market Sector [Member] | Business Transformation Services [Member] | Automotive Market Sector [Member]
Disaggregation of Revenue [Line Items]
Revenue		$ 155,105	$ 105,431