New Accounting Pronouncements - Summary of Impact of Adopting New Accounting Standards on Consolidated Financial Statements (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Accounts receivable, net	$ 234,906		$ 234,906			$ 188,300
Other current assets	188,931		188,931
Other non-current assets	766,019		766,019
Total assets	1,189,856		1,189,856			1,176,231
Accounts payable	57,764		57,764			51,375
Accrued personnel costs	55,038		55,038			45,264
Other current liabilities	168,995		168,995
Deferred income taxes, net	5,636		5,636			3,339
Other non-current liabilities	300,508		300,508
Total liabilities	587,941		587,941			645,352
Retained earnings	409,467		409,467			345,302
Other stockholders' equity	192,448		192,448
Total stockholders’ equity	601,915		601,915			530,879
Total liabilities and stockholders’ equity	1,189,856		1,189,856			1,176,231
Revenue	224,249	$ 207,723	722,980	$ 660,198
Operating expenses	198,607	184,723	608,459	565,609
Gross margin	25,642	23,000	114,521	94,589
Corporate general and administrative expenses	10,279	7,979	30,300	25,979
Operating income (loss)	15,363	15,021	84,221	68,610
Other (expense) income:
Interest expense	(1,614)	(1,777)	(5,211)	(4,986)
Gain on sale of operations, net			663	45
Other income, net	3,143	2,792	2,544	9,293
Total other income (expense), net	1,529	1,015	(2,004)	4,352
Income from continuing operations before income tax expense	16,892	16,036	82,217	72,962
Income tax expense	3,297	6,172	19,691	26,656
Income from continuing operations	13,595	9,864	62,526	46,306
(Loss) gain from discontinued operations, net of tax	(9)	(206)	17	(776)
Net income	13,586	9,658	62,543	45,530
Cash flows from operating activities:
Net income	13,586	9,658	62,543	45,530
Adjustments to reconcile net income to net cash provided by operating activities:			27,884
Changes in assets and liabilities, net of acquisitions and divestitures:
Accounts receivable, net			(38,937)	(38,095)
Other assets			(3,474)	2,293
Accounts payable			108	(61)
Accrued personnel costs			8,986	(7,368)
Other liabilities			(3,134)	1,606
Other			9,458
Operating cash flows provide by continuing operations			63,434	38,599
Operating cash flows used in discontinued operations			(162)	(748)
Net cash provided by operating activities			63,272	37,851
Net provided by investing activities			42,201	43,463
Net cash used in financing activities			(102,838)	(79,306)
Net increase in cash, cash equivalents and restricted cash			2,635
Cash, cash equivalents and restricted cash at beginning of year			33,409	31,374
Cash, cash equivalents and restricted cash at end of period	36,044	$ 33,382	36,044	$ 33,382
Previously Reported [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Accounts receivable, net						188,300
Other current assets						259,873
Other non-current assets						728,058
Total assets						1,176,231
Accounts payable						51,375
Accrued personnel costs						45,264
Other current liabilities						237,607
Deferred income taxes, net						3,339
Other non-current liabilities						307,767
Total liabilities						645,352
Retained earnings						345,302
Other stockholders' equity						185,577
Total stockholders’ equity						530,879
Total liabilities and stockholders’ equity						1,176,231
Balances without Adoption of Topic 606 [Member] | Previously Reported [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Accounts receivable, net	219,809		219,809
Other current assets	188,851		188,851
Other non-current assets	765,352		765,352
Total assets	1,174,012		1,174,012
Accounts payable	47,488		47,488
Accrued personnel costs	54,463		54,463
Other current liabilities	168,881		168,881
Deferred income taxes, net	4,598		4,598
Other non-current liabilities	299,583		299,583
Total liabilities	575,013		575,013
Retained earnings	406,551		406,551
Other stockholders' equity	192,448		192,448
Total stockholders’ equity	598,999		598,999
Total liabilities and stockholders’ equity	1,174,012		1,174,012
Revenue	223,486		721,236
Operating expenses	198,593		608,416
Gross margin	24,893		112,820
Corporate general and administrative expenses	10,279		30,300
Operating income (loss)	14,614		82,520
Other (expense) income:
Interest expense	(1,614)		(5,211)
Gain on sale of operations, net			663
Other income, net	3,143		2,544
Total other income (expense), net	1,529		(2,004)
Income from continuing operations before income tax expense	16,143		80,516
Income tax expense	3,129		19,284
Income from continuing operations	13,014		61,232
(Loss) gain from discontinued operations, net of tax	(9)		17
Net income	13,005		61,249
Cash flows from operating activities:
Net income	13,005		61,249
Adjustments to reconcile net income to net cash provided by operating activities:			27,884
Changes in assets and liabilities, net of acquisitions and divestitures:
Accounts receivable, net			(33,286)
Other assets			(3,535)
Accounts payable			(3,887)
Accrued personnel costs			9,006
Other liabilities			(3,455)
Other			9,458
Operating cash flows provide by continuing operations			63,434
Operating cash flows used in discontinued operations			(162)
Net cash provided by operating activities			63,272
Net provided by investing activities			42,201
Net cash used in financing activities			(102,838)
Net increase in cash, cash equivalents and restricted cash			2,635
Cash, cash equivalents and restricted cash at beginning of year			33,409
Cash, cash equivalents and restricted cash at end of period	36,044		36,044
Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Accounts receivable, net					$ 197,746
Other current assets					259,953
Other non-current assets					728,786
Total assets					1,186,485
Accounts payable					57,656
Accrued personnel costs					45,859
Other current liabilities					237,720
Deferred income taxes, net					3,970
Other non-current liabilities					308,779
Total liabilities					653,984
Retained earnings					346,924
Other stockholders' equity					185,577
Total stockholders’ equity					532,501	$ 532,501
Total liabilities and stockholders’ equity					1,186,485
Accounting Standards Update 2014-09 [Member] | Adjustments [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Accounts receivable, net					9,446
Other current assets					80
Other non-current assets					728
Total assets					10,254
Accounts payable					6,281
Accrued personnel costs					595
Other current liabilities					113
Deferred income taxes, net					631
Other non-current liabilities					1,012
Total liabilities					8,632
Retained earnings					1,622
Total stockholders’ equity					1,622
Total liabilities and stockholders’ equity					$ 10,254
Accounting Standards Update 2014-09 [Member] | Adjustments [Member] | Adjustments [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Accounts receivable, net	(15,097)		(15,097)
Other current assets	(80)		(80)
Other non-current assets	(667)		(667)
Total assets	(15,844)		(15,844)
Accounts payable	(10,276)		(10,276)
Accrued personnel costs	(575)		(575)
Other current liabilities	(114)		(114)
Deferred income taxes, net	(1,038)		(1,038)
Other non-current liabilities	(925)		(925)
Total liabilities	(12,928)		(12,928)
Retained earnings	(2,916)		(2,916)
Total stockholders’ equity	(2,916)		(2,916)
Total liabilities and stockholders’ equity	(15,844)		(15,844)
Revenue	(763)		(1,744)
Operating expenses	(14)		(43)
Gross margin	(749)		(1,701)
Operating income (loss)	(749)		(1,701)
Other (expense) income:
Income from continuing operations before income tax expense	(749)		(1,701)
Income tax expense	(168)		(407)
Income from continuing operations	(581)		(1,294)
Net income	(581)		(1,294)
Cash flows from operating activities:
Net income	$ (581)		(1,294)
Changes in assets and liabilities, net of acquisitions and divestitures:
Accounts receivable, net			5,651
Other assets			(61)
Accounts payable			(3,995)
Accrued personnel costs			20
Other liabilities			$ (321)