Summary of Significant Accounting Policies - Summary of Impact of Adoption of Accounting Standards (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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	Jan. 01, 2018	Dec. 31, 2015
Revenue:
Total revenue	$ 192,115	$ 179,253	$ 170,144	$ 163,077	$ 112,682	$ 110,643	$ 106,216	$ 106,611	$ 704,589	$ 436,152	$ 421,582
Operating expenses:
Direct service costs, excluding depreciation and amortization									252,284	211,773	198,510
Reimbursed out-of-pocket expenses									236,775	49,690	50,961
Total direct costs	131,133	123,996	116,676	117,254	68,803	65,106	63,619	63,935	489,059	261,463	249,471
Total operating expenses									603,541	371,294	369,092
Income from operations	30,666	26,918	23,345	20,119	15,437	17,198	16,279	15,944	101,048	64,858	52,490
Income before income taxes									93,951	56,945	21,957
Income tax provision									20,766	17,823	8,532
Net income	$ 22,761	$ 19,305	$ 16,568	$ 14,551	$ 11,291	$ 9,831	$ 9,553	$ 8,447	$ 73,185	$ 39,122	$ 13,425
Net income per share attributable to common shareholders:
Basic	$ 0.64	$ 0.54	$ 0.46	$ 0.41	$ 0.30	$ 0.25	$ 0.24	$ 0.21	$ 2.05	$ 1.00	$ 0.38
Diluted	$ 0.61	$ 0.52	$ 0.45	$ 0.40	$ 0.30	$ 0.25	$ 0.23	$ 0.20	$ 1.97	$ 0.98	$ 0.37
Weighted average common shares outstanding:
Basic									35,547	39,056	35,690
Diluted									36,912	39,839	36,329
Current assets:
Accounts receivable and unbilled, net	$ 133,449				$ 83,079				$ 133,449	$ 83,079
Prepaid expenses and other current assets	21,383				20,400				21,383	20,400
Total current assets	178,114				129,971				178,114	129,971
Deferred income taxes	713				6,343				713	6,343
Total assets	967,933				950,717				967,933	950,717
Current liabilities:
Accrued expenses	87,493				23,673				87,493	23,673
Pre-funded study costs	0				57,406				0	57,406
Advanced billings	147,935				73,756				147,935	73,756
Other current liabilities	4,861				4,697				4,861	4,697
Total current liabilities	257,026				192,706				257,026	192,706
Deferred income tax liability	439				560				439	560
Other long-term liabilities	12,804				10,740				12,804	10,740
Total liabilities	378,230				447,187				378,230	447,187
Shareholders’ equity:
Accumulated deficit	(41,487)				(120,402)				(41,487)	(120,402)
Total shareholders’ equity	589,703				503,530				589,703	503,530	$ 610,710		$ 413,474
Total liabilities and shareholders’ equity	967,933				$ 950,717				967,933	950,717
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income									73,185	39,122	13,425
Adjustments to reconcile net income to net cash provided by operating activities:
Deferred income tax provision (benefit)									3,942	3,237	(9,006)
Changes in assets and liabilities:
Accounts receivable and unbilled, net									(27,047)	(2,898)	(13,727)
Prepaid expenses and other current assets									(1,241)	(3,533)	(3,661)
Accrued expenses									29,029	(1,313)	4,516
Pre-funded study costs									0	5,292	5,400
Advanced billings									35,593	7,735	14,723
Other assets and liabilities, net									1,925	2,782	(841)
Net cash provided by operating activities									156,584	97,385	91,732
Revenue Net
Revenue:
Total revenue									704,589	0	0
Direct Service Revenue
Revenue:
Total revenue									0	386,462	370,621
Reimbursed Out-of-Pocket Revenue
Revenue:
Total revenue									0	$ 49,690	$ 50,961
ASC 606
Current assets:
Accounts receivable and unbilled, net												$ 106,966
Current liabilities:
Pre-funded study costs												0
Advanced billings												112,613
ASC 606 | Impact of Adoption of ASC 606 - Adjustments
Revenue:
Total revenue									(155,221)
Operating expenses:
Direct service costs, excluding depreciation and amortization									0
Reimbursed out-of-pocket expenses									(165,470)
Total direct costs									(165,470)
Total operating expenses									(165,470)
Income from operations									10,249
Income before income taxes									10,249
Income tax provision									1,882
Net income									$ 8,367
Net income per share attributable to common shareholders:
Basic									$ 0.24
Diluted									$ 0.23
Weighted average common shares outstanding:
Basic									0
Diluted									0
Current assets:
Accounts receivable and unbilled, net	(28,729)								$ (28,729)			23,887
Prepaid expenses and other current assets	1,147								1,147
Total current assets	(27,582)								(27,582)
Deferred income taxes	(389)								(389)
Total assets	(27,971)								(27,971)
Current liabilities:
Accrued expenses	(51,109)								(51,109)			35,100
Pre-funded study costs	61,156								61,156			(57,406)
Advanced billings	(41,732)								(41,732)			38,857
Other current liabilities	(590)								(590)
Total current liabilities	(32,275)								(32,275)
Deferred income tax liability	2,049								2,049			(1,600)
Other long-term liabilities	(382)								(382)
Total liabilities	(30,608)								(30,608)
Shareholders’ equity:
Accumulated deficit	2,637								2,637			$ 5,700
Total shareholders’ equity	2,637								2,637
Total liabilities and shareholders’ equity	(27,971)								(27,971)
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income									8,367
Adjustments to reconcile net income to net cash provided by operating activities:
Deferred income tax provision (benefit)									4,002
Changes in assets and liabilities:
Accounts receivable and unbilled, net									4,842
Prepaid expenses and other current assets									(1,147)
Accrued expenses									(15,967)
Pre-funded study costs									3,782
Advanced billings									(2,907)
Other assets and liabilities, net									(972)
Net cash provided by operating activities									0
ASC 606 | Impact of Adoption of ASC 606 - Adjustments | Revenue Net
Revenue:
Total revenue									(704,589)
ASC 606 | Impact of Adoption of ASC 606 - Adjustments | Direct Service Revenue
Revenue:
Total revenue									478,063
ASC 606 | Impact of Adoption of ASC 606 - Adjustments | Reimbursed Out-of-Pocket Revenue
Revenue:
Total revenue									71,305
ASC 606 | Impact of Adoption of ASC 606 - As Revised under ASC 605
Revenue:
Total revenue									549,368
Operating expenses:
Direct service costs, excluding depreciation and amortization									252,284
Reimbursed out-of-pocket expenses									71,305
Total direct costs									323,589
Total operating expenses									438,071
Income from operations									111,297
Income before income taxes									104,200
Income tax provision									22,648
Net income									$ 81,552
Net income per share attributable to common shareholders:
Basic									$ 2.29
Diluted									$ 2.20
Weighted average common shares outstanding:
Basic									35,547
Diluted									36,912
Current assets:
Accounts receivable and unbilled, net	104,720								$ 104,720
Prepaid expenses and other current assets	22,530								22,530
Total current assets	150,532								150,532
Deferred income taxes	324								324
Total assets	939,962								939,962
Current liabilities:
Accrued expenses	36,384								36,384
Pre-funded study costs	61,156								61,156
Advanced billings	106,203								106,203
Other current liabilities	4,271								4,271
Total current liabilities	224,751								224,751
Deferred income tax liability	2,488								2,488
Other long-term liabilities	12,422								12,422
Total liabilities	347,622								347,622
Shareholders’ equity:
Accumulated deficit	(38,850)								(38,850)
Total shareholders’ equity	592,340								592,340
Total liabilities and shareholders’ equity	$ 939,962								939,962
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income									81,552
Adjustments to reconcile net income to net cash provided by operating activities:
Deferred income tax provision (benefit)									7,944
Changes in assets and liabilities:
Accounts receivable and unbilled, net									(22,205)
Prepaid expenses and other current assets									(2,388)
Accrued expenses									13,062
Pre-funded study costs									3,782
Advanced billings									32,686
Other assets and liabilities, net									953
Net cash provided by operating activities									156,584
ASC 606 | Impact of Adoption of ASC 606 - As Revised under ASC 605 | Revenue Net
Revenue:
Total revenue									0
ASC 606 | Impact of Adoption of ASC 606 - As Revised under ASC 605 | Direct Service Revenue
Revenue:
Total revenue									478,063
ASC 606 | Impact of Adoption of ASC 606 - As Revised under ASC 605 | Reimbursed Out-of-Pocket Revenue
Revenue:
Total revenue									$ 71,305