Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Services	$ 375,830	$ 375,800	$ 1,082,541	$ 1,111,032
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Disaggregation of Revenue [Line Items]
Services	8,172		39,176
Calculated under Revenue Guidance in Effect before Topic 606
Disaggregation of Revenue [Line Items]
Services	384,002		1,121,717
United States
Disaggregation of Revenue [Line Items]
Services	296,544	289,701	848,336	868,847
United States | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Disaggregation of Revenue [Line Items]
Services	1,899		16,940
United States | Calculated under Revenue Guidance in Effect before Topic 606
Disaggregation of Revenue [Line Items]
Services	298,443		865,276
Canada
Disaggregation of Revenue [Line Items]
Services	32,132	31,418	91,597	88,471
Canada | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Disaggregation of Revenue [Line Items]
Services	286		(2,352)
Canada | Calculated under Revenue Guidance in Effect before Topic 606
Disaggregation of Revenue [Line Items]
Services	32,418		89,245
Other
Disaggregation of Revenue [Line Items]
Services	47,154	54,681	142,608	153,714
Other | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Disaggregation of Revenue [Line Items]
Services	5,987		24,588
Other | Calculated under Revenue Guidance in Effect before Topic 606
Disaggregation of Revenue [Line Items]
Services	53,141		167,196
Food & Beverage
Disaggregation of Revenue [Line Items]
Services	80,919	80,247	234,203	225,873
Food & Beverage | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Disaggregation of Revenue [Line Items]
Services	(1,212)		4,171
Food & Beverage | Calculated under Revenue Guidance in Effect before Topic 606
Disaggregation of Revenue [Line Items]
Services	79,707		238,374
Retail
Disaggregation of Revenue [Line Items]
Services	40,421	43,202	116,832	134,993
Retail | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Disaggregation of Revenue [Line Items]
Services	(4,457)		(2,685)
Retail | Calculated under Revenue Guidance in Effect before Topic 606
Disaggregation of Revenue [Line Items]
Services	35,964		114,147
Consumer Products
Disaggregation of Revenue [Line Items]
Services	40,124	43,825	118,097	119,554
Consumer Products | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Disaggregation of Revenue [Line Items]
Services	1,136		362
Consumer Products | Calculated under Revenue Guidance in Effect before Topic 606
Disaggregation of Revenue [Line Items]
Services	41,260		118,459
Communications
Disaggregation of Revenue [Line Items]
Services	46,779	46,649	128,232	150,703
Communications | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Disaggregation of Revenue [Line Items]
Services	8,337		20,519
Communications | Calculated under Revenue Guidance in Effect before Topic 606
Disaggregation of Revenue [Line Items]
Services	55,116		148,751
Automotive
Disaggregation of Revenue [Line Items]
Services	21,282	30,547	67,070	96,832
Automotive | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Disaggregation of Revenue [Line Items]
Services	734		6,809
Automotive | Calculated under Revenue Guidance in Effect before Topic 606
Disaggregation of Revenue [Line Items]
Services	22,016		73,879
Technology
Disaggregation of Revenue [Line Items]
Services	26,005	25,748	71,085	72,620
Technology | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Disaggregation of Revenue [Line Items]
Services	171		(139)
Technology | Calculated under Revenue Guidance in Effect before Topic 606
Disaggregation of Revenue [Line Items]
Services	26,176		70,946
Healthcare
Disaggregation of Revenue [Line Items]
Services	33,751	31,181	101,753	92,380
Healthcare | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Disaggregation of Revenue [Line Items]
Services	84		603
Healthcare | Calculated under Revenue Guidance in Effect before Topic 606
Disaggregation of Revenue [Line Items]
Services	33,835		102,356
Financials
Disaggregation of Revenue [Line Items]
Services	30,378	26,631	83,079	73,777
Financials | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Disaggregation of Revenue [Line Items]
Services	283		1,194
Financials | Calculated under Revenue Guidance in Effect before Topic 606
Disaggregation of Revenue [Line Items]
Services	30,661		84,273
Transportation and Travel/Lodging
Disaggregation of Revenue [Line Items]
Services	19,357	14,412	53,021	42,187
Transportation and Travel/Lodging | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Disaggregation of Revenue [Line Items]
Services	1,333		2,109
Transportation and Travel/Lodging | Calculated under Revenue Guidance in Effect before Topic 606
Disaggregation of Revenue [Line Items]
Services	20,690		55,130
Other
Disaggregation of Revenue [Line Items]
Services	36,814	$ 33,358	109,169	$ 102,113
Other | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Disaggregation of Revenue [Line Items]
Services	1,763		6,233
Other | Calculated under Revenue Guidance in Effect before Topic 606
Disaggregation of Revenue [Line Items]
Services	$ 38,577		$ 115,402