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
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue:
Revenue, net	$ 163,077	$ 0
Service revenue, net	0	93,781
Reimbursed out-of-pocket revenue	0	12,830
Total revenue	163,077	106,611
Operating expenses:
Direct service costs, excluding depreciation and amortization	60,341	51,105
Reimbursed out-of-pocket expenses	56,913	12,830
Total direct costs	117,254	63,935
Total operating expenses	142,958	90,667
Income from operations	20,119	15,944
Income before income taxes	17,657	13,778
Income tax provision	3,106	5,331
Net income	$ 14,551	$ 8,447
Net income per share attributable to common shareholders:
Basic	$ 0.41	$ 0.21
Diluted	$ 0.40	$ 0.20
Weighted average common shares outstanding:
Basic	35,486	40,669
Diluted	36,449	41,508
Current assets:
Accounts receivable and unbilled, net	$ 110,654			$ 83,079
Total current assets	153,754			129,971
Deferred income taxes	9,970			6,343
Total assets	973,024			950,717
Current liabilities:
Accrued expenses	63,576			23,673
Pre-funded study costs	0			57,406
Advanced billings	126,387			73,756
Other current liabilities	8,105			4,697
Total current liabilities	232,492			192,706
Total liabilities	461,951			447,187
Shareholders’ equity:
Accumulated deficit	(115,521)			(120,402)
Total shareholders’ equity	511,073			503,530
Total liabilities and shareholders’ equity	973,024			$ 950,717
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	14,551	$ 8,447
Changes in assets and liabilities:
Accounts receivable and unbilled, net	(17,760)	(2,292)
Accrued expenses	5,268	(10,508)
Pre-funded study costs	0	(3,497)
Advanced billings	6,817	(5,246)
Other assets and liabilities, net	3,089	1,884
Net cash (used in) provided by operating activities	23,298	$ (675)
ASC 606
Current assets:
Accounts receivable and unbilled, net			$ 92,832
Current liabilities:
Advanced billings			119,451
ASC 606 | Impact of Adoption of ASC 606 - Adjustments
Revenue:
Revenue, net	(163,077)
Service revenue, net	108,428
Reimbursed out-of-pocket revenue	15,017
Total revenue	(39,632)
Operating expenses:
Reimbursed out-of-pocket expenses	(41,896)
Total direct costs	(41,896)
Total operating expenses	(41,896)
Income from operations	2,264
Income before income taxes	2,264
Income tax provision	511
Net income	$ 1,753
Net income per share attributable to common shareholders:
Basic	$ 0.05
Diluted	$ 0.05
Current assets:
Accounts receivable and unbilled, net	$ (12,733)		9,753
Total current assets	(12,733)
Deferred income taxes	(3,347)		3,300
Total assets	(16,080)
Current liabilities:
Accrued expenses	(45,145)		34,500
Pre-funded study costs	57,487		(57,406)
Advanced billings	(40,356)		45,695
Other current liabilities	511
Total current liabilities	(27,503)
Total liabilities	(27,503)
Shareholders’ equity:
Accumulated deficit	11,423		$ (9,700)
Total shareholders’ equity	11,423
Total liabilities and shareholders’ equity	(16,080)
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	1,753
Changes in assets and liabilities:
Accounts receivable and unbilled, net	2,980
Accrued expenses	(10,664)
Pre-funded study costs	50
Advanced billings	5,370
Other assets and liabilities, net	511
ASC 606 | Impact of Adoption of ASC 606 - As Revised under ASC 605
Revenue:
Revenue, net	0
Service revenue, net	108,428
Reimbursed out-of-pocket revenue	15,017
Total revenue	123,445
Operating expenses:
Direct service costs, excluding depreciation and amortization	60,341
Reimbursed out-of-pocket expenses	15,017
Total direct costs	75,358
Total operating expenses	101,062
Income from operations	22,383
Income before income taxes	19,921
Income tax provision	3,617
Net income	$ 16,304
Net income per share attributable to common shareholders:
Basic	$ 0.46
Diluted	$ 0.45
Weighted average common shares outstanding:
Basic	35,486
Diluted	36,449
Current assets:
Accounts receivable and unbilled, net	$ 97,921
Total current assets	141,021
Deferred income taxes	6,623
Total assets	956,944
Current liabilities:
Accrued expenses	18,431
Pre-funded study costs	57,487
Advanced billings	86,031
Other current liabilities	8,616
Total current liabilities	204,989
Total liabilities	434,448
Shareholders’ equity:
Accumulated deficit	(104,098)
Total shareholders’ equity	522,496
Total liabilities and shareholders’ equity	956,944
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	16,304
Changes in assets and liabilities:
Accounts receivable and unbilled, net	(14,780)
Accrued expenses	(5,396)
Pre-funded study costs	50
Advanced billings	12,187
Other assets and liabilities, net	3,600
Net cash (used in) provided by operating activities	$ 23,298