CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Revenues	$ 1,859,922us-gaap_SalesRevenueServicesNet	$ 1,628,927us-gaap_SalesRevenueServicesNet	$ 1,134,614us-gaap_SalesRevenueServicesNet
Cost of services	1,262,010us-gaap_CostOfServices	1,143,438us-gaap_CostOfServices	781,905us-gaap_CostOfServices
Gross profit	597,912us-gaap_GrossProfit	485,489us-gaap_GrossProfit	352,709us-gaap_GrossProfit
Selling, general and administrative expenses	427,241	339,423	252,574
Amortization of intangible assets	24,401us-gaap_AmortizationOfIntangibleAssets	21,686us-gaap_AmortizationOfIntangibleAssets	17,863us-gaap_AmortizationOfIntangibleAssets
Operating income	146,270us-gaap_OperatingIncomeLoss	124,380us-gaap_OperatingIncomeLoss	82,272us-gaap_OperatingIncomeLoss
Interest expense, net	(12,730)us-gaap_InterestExpense	(15,863)us-gaap_InterestExpense	(15,768)us-gaap_InterestExpense
Write-off of loan costs	0	(14,958)	(813)
Income before income taxes	133,540	93,559	65,691
Provision for income taxes	54,527us-gaap_IncomeTaxExpenseBenefit	38,848us-gaap_IncomeTaxExpenseBenefit	28,496us-gaap_IncomeTaxExpenseBenefit
Income from continuing operations	79,013	54,711	37,195
Gain on sale of discontinued operations, net of income taxes	0	30,840	0
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent	(1,829)	(1,039)	5,458
Net income	77,184us-gaap_NetIncomeLoss	84,512us-gaap_NetIncomeLoss	42,653us-gaap_NetIncomeLoss
Basic earnings per common share:
Continuing operations	$ 1.48	$ 1.02	$ 0.80
Discontinued operations	$ (0.04)	$ 0.56	$ 0.11
Net income	$ 1.44us-gaap_EarningsPerShareBasic	$ 1.58us-gaap_EarningsPerShareBasic	$ 0.91us-gaap_EarningsPerShareBasic
Diluted earnings per common share:
Continuing operations	$ 1.46	$ 1.00	$ 0.78
Discontinued operations	$ (0.04)	$ 0.55	$ 0.11
Net income	$ 1.42us-gaap_EarningsPerShareDiluted	$ 1.55us-gaap_EarningsPerShareDiluted	$ 0.89us-gaap_EarningsPerShareDiluted
Number of shares and share equivalents used to calculate earnings per share:
Basic (in shares)	53,437	53,481	46,739
Diluted (in shares)	54,294	54,555	47,826
Reconciliation of net income to comprehensive income:
Net income	77,184us-gaap_NetIncomeLoss	84,512us-gaap_NetIncomeLoss	42,653us-gaap_NetIncomeLoss
Changes in fair value of derivative, net of tax	86	193	(21)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Gain (Loss) Arising During Period, Net of Tax	(4,772)	1,128	854
Comprehensive income	$ 72,498us-gaap_ComprehensiveIncomeNetOfTax	$ 85,833us-gaap_ComprehensiveIncomeNetOfTax	$ 43,486us-gaap_ComprehensiveIncomeNetOfTax

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The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.