Unaudited Quarterly Financial Results (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended	12 Months Ended
Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 441,023us-gaap_SalesRevenueServicesNet	$ 442,443us-gaap_SalesRevenueServicesNet	$ 434,424us-gaap_SalesRevenueServicesNet	$ 406,851us-gaap_SalesRevenueServicesNet	$ 396,070us-gaap_SalesRevenueServicesNet	$ 394,400us-gaap_SalesRevenueServicesNet	$ 380,779us-gaap_SalesRevenueServicesNet	$ 351,852us-gaap_SalesRevenueServicesNet	$ 1,724,741us-gaap_SalesRevenueServicesNet	$ 1,523,101us-gaap_SalesRevenueServicesNet	$ 1,031,935us-gaap_SalesRevenueServicesNet
Gross profit	142,537us-gaap_GrossProfit	144,838us-gaap_GrossProfit	141,905us-gaap_GrossProfit	128,155us-gaap_GrossProfit	120,985us-gaap_GrossProfit	119,016us-gaap_GrossProfit	113,105us-gaap_GrossProfit	101,769us-gaap_GrossProfit	557,435us-gaap_GrossProfit	454,875us-gaap_GrossProfit	321,254us-gaap_GrossProfit
Income from continuing operations	19,526	20,723	19,641	13,605	16,321	18,656	6,649	9,514	73,495	51,140	33,463
Income from discontinued operations, net of income taxes	947	1,282	1,148	312	16,089	1,494	690	15,099	3,689	2,532	9,190
Net income	20,473us-gaap_NetIncomeLoss	22,005us-gaap_NetIncomeLoss	20,789us-gaap_NetIncomeLoss	13,917us-gaap_NetIncomeLoss	32,410us-gaap_NetIncomeLoss	20,150us-gaap_NetIncomeLoss	7,339us-gaap_NetIncomeLoss	24,613us-gaap_NetIncomeLoss	77,184us-gaap_NetIncomeLoss	84,512us-gaap_NetIncomeLoss	42,653us-gaap_NetIncomeLoss
Earnings Per Share, Basic [Abstract]
Continuing operations	$ 0.38	$ 0.39	$ 0.36	$ 0.25	$ 0.30	$ 0.35	$ 0.12	$ 0.18	$ 1.38	$ 0.96	$ 0.72
Discontinued operations	$ 0.01	$ 0.02	$ 0.02	$ 0.01	$ 0.30	$ 0.03	$ 0.02	$ 0.28	$ 0.06	$ 0.62	$ 0.19
Net income	$ 0.39us-gaap_EarningsPerShareBasic	$ 0.41us-gaap_EarningsPerShareBasic	$ 0.38us-gaap_EarningsPerShareBasic	$ 0.26us-gaap_EarningsPerShareBasic	$ 0.60us-gaap_EarningsPerShareBasic	$ 0.38us-gaap_EarningsPerShareBasic	$ 0.14us-gaap_EarningsPerShareBasic	$ 0.46us-gaap_EarningsPerShareBasic	$ 1.44us-gaap_EarningsPerShareBasic	$ 1.58us-gaap_EarningsPerShareBasic	$ 0.91us-gaap_EarningsPerShareBasic
Earnings Per Share, Diluted [Abstract]
Continuing operations	$ 0.37	$ 0.38	$ 0.36	$ 0.25	$ 0.30	$ 0.34	$ 0.12	$ 0.18	$ 1.35	$ 0.94	$ 0.70
Discontinued operations	$ 0.02	$ 0.03	$ 0.02	$ 0.00	$ 0.29	$ 0.03	$ 0.02	$ 0.28	$ 0.07	$ 0.61	$ 0.19
Net income	$ 0.39us-gaap_EarningsPerShareDiluted	$ 0.41us-gaap_EarningsPerShareDiluted	$ 0.38us-gaap_EarningsPerShareDiluted	$ 0.25us-gaap_EarningsPerShareDiluted	$ 0.59us-gaap_EarningsPerShareDiluted	$ 0.37us-gaap_EarningsPerShareDiluted	$ 0.14us-gaap_EarningsPerShareDiluted	$ 0.46us-gaap_EarningsPerShareDiluted	$ 1.42us-gaap_EarningsPerShareDiluted	$ 1.55us-gaap_EarningsPerShareDiluted	$ 0.89us-gaap_EarningsPerShareDiluted
Apex Segment [Member]
Quarterly Financial Information Disclosure [Abstract]
Revenues									1,190,052	1,059,993	617,844
Gross profit									$ 335,322	$ 294,611	$ 174,968

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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