Segments - Financial Information Attributable to the Company's Business Segments (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Service revenue, net									$ 1,384,965	$ 1,318,220	$ 1,233,842
Service expense									1,284,603	1,223,627	1,131,963
General and administrative expense									46,098	43,491	39,527
Asset impairment charge									23,378	0	21,003
Depreciation and amortization									15,813	13,618	12,780
Operating income	$ (693)	$ 9,435	$ 3,431	$ 12,103	$ 14,173	$ 7,271	$ 11,333	$ 4,707	24,276	37,484	48,157
Equity in net (gain) loss of investees									6,072	(12,054)	10,287
Investment in equity method investee	161,503				169,699				161,503	169,699
Total assets	572,246				704,090				572,246	704,090
NET Services
Segment Reporting Information [Line Items]
Service revenue, net									1,384,965
Asset impairment charge	13,496
Continuing Operations
Segment Reporting Information [Line Items]
Service revenue, net									1,384,965	1,318,220	1,233,842
Service expense									1,284,603	1,223,627	1,131,963
General and administrative expense									46,098	43,491	39,527
Asset impairment charge									14,175	0	1,415
Depreciation and amortization									15,813	13,618	12,780
Operating income									24,276	37,484	48,157
Equity in net (gain) loss of investees									6,158	(13,445)	1,789
Investment in equity method investee	161,503				169,699				161,503	169,699
Total assets	565,195				526,238				565,195	526,238
Long-lived asset expenditures									10,796	15,396	12,232
Continuing Operations | Corporate and Other
Segment Reporting Information [Line Items]
Service revenue, net									0	0	122
Service expense									(426)	(3,799)	(894)
General and administrative expense									31,851	31,712	28,121
Asset impairment charge									0	0	1,415
Depreciation and amortization									787	343	405
Operating income									(32,212)	(28,256)	(28,925)
Equity in net (gain) loss of investees									0	0	0
Investment in equity method investee	0								0
Total assets	54,125				62,412				54,125	62,412
Long-lived asset expenditures									0	77	1,387
Continuing Operations | NET Services | Operating Segments
Segment Reporting Information [Line Items]
Service revenue, net									1,384,965	1,318,220	1,233,720
Service expense									1,285,029	1,227,426	1,132,857
General and administrative expense									14,247	11,779	11,406
Asset impairment charge									14,175	0	0
Depreciation and amortization									15,026	13,275	12,375
Operating income									56,488	65,740	77,082
Equity in net (gain) loss of investees									0	0	0
Investment in equity method investee	0								0
Total assets	349,567				294,127				349,567	294,127
Long-lived asset expenditures									10,796	15,319	10,845
Continuing Operations | Matrix Investment | Operating Segments
Segment Reporting Information [Line Items]
Service revenue, net									0	0	0
Service expense									0	0	0
General and administrative expense									0	0	0
Asset impairment charge									0	0	0
Depreciation and amortization									0	0	0
Operating income									0	0	0
Equity in net (gain) loss of investees									6,158	(13,445)	1,789
Investment in equity method investee	161,503				169,699				161,503	169,699
Total assets	$ 161,503				$ 169,699				161,503	169,699
Long-lived asset expenditures									$ 0	$ 0	$ 0