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	$ 475,808us-gaap_SalesRevenueServicesNet	$ 477,391us-gaap_SalesRevenueServicesNet	$ 468,081us-gaap_SalesRevenueServicesNet	$ 438,642us-gaap_SalesRevenueServicesNet	$ 422,906us-gaap_SalesRevenueServicesNet	$ 420,623us-gaap_SalesRevenueServicesNet	$ 407,244us-gaap_SalesRevenueServicesNet	$ 378,154us-gaap_SalesRevenueServicesNet	$ 1,859,922us-gaap_SalesRevenueServicesNet	$ 1,628,927us-gaap_SalesRevenueServicesNet	$ 1,134,614us-gaap_SalesRevenueServicesNet
Gross profit	153,534us-gaap_GrossProfit	155,182us-gaap_GrossProfit	152,203us-gaap_GrossProfit	136,993us-gaap_GrossProfit	129,094us-gaap_GrossProfit	126,398us-gaap_GrossProfit	120,745us-gaap_GrossProfit	109,252us-gaap_GrossProfit	597,912us-gaap_GrossProfit	485,489us-gaap_GrossProfit	352,709us-gaap_GrossProfit
Income from continuing operations	21,790	22,238	20,838	14,147	17,487	19,603	7,510	10,111	79,013	54,711	37,195
Income from discontinued operations, net of income taxes	(1,317)	(233)	(49)	(230)	14,923	547	(171)	14,502	(1,829)	(1,039)	5,458
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	$ 0.42	$ 0.42	$ 0.38	$ 0.26	$ 0.32	$ 0.37	$ 0.14	$ 0.19	$ 1.48	$ 1.02	$ 0.80
Discontinued operations	$ (0.03)	$ (0.01)	$ 0.00	$ 0.00	$ 0.28	$ 0.01	$ 0.00	$ 0.27	$ (0.04)	$ 0.56	$ 0.11
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	$ 0.41	$ 0.41	$ 0.38	$ 0.26	$ 0.32	$ 0.36	$ 0.14	$ 0.19	$ 1.46	$ 1.00	$ 0.78
Discontinued operations	$ (0.02)	$ 0.00	$ 0.00	$ (0.01)	$ 0.27	$ 0.01	$ 0.00	$ 0.27	$ (0.04)	$ 0.55	$ 0.11
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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