UNAUDITED QUARTERLY DATA (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
UNAUDITED QUARTERLY DATA
Revenues	$ 343,951us-gaap_SalesRevenueServicesNet	$ 384,870us-gaap_SalesRevenueServicesNet	$ 369,357us-gaap_SalesRevenueServicesNet	$ 313,388us-gaap_SalesRevenueServicesNet	$ 324,707us-gaap_SalesRevenueServicesNet	$ 362,155us-gaap_SalesRevenueServicesNet	$ 350,798us-gaap_SalesRevenueServicesNet	$ 299,714us-gaap_SalesRevenueServicesNet	$ 1,411,566us-gaap_SalesRevenueServicesNet	$ 1,337,374us-gaap_SalesRevenueServicesNet	$ 1,270,909us-gaap_SalesRevenueServicesNet
Gross profit (Revenues less cost of services provided)	168,972us-gaap_GrossProfit	196,060us-gaap_GrossProfit	186,715us-gaap_GrossProfit	152,080us-gaap_GrossProfit	157,350us-gaap_GrossProfit	181,020us-gaap_GrossProfit	176,437us-gaap_GrossProfit	144,108us-gaap_GrossProfit
Net Income	$ 29,917	$ 41,121	$ 40,860	$ 25,766	$ 27,957	$ 36,200	$ 35,994	$ 23,179	$ 137,664	$ 123,330	$ 111,332
Income per share:
Income per share Basic (in dollars per share)	$ 0.21us-gaap_EarningsPerShareBasic	$ 0.28us-gaap_EarningsPerShareBasic	$ 0.28us-gaap_EarningsPerShareBasic	$ 0.18us-gaap_EarningsPerShareBasic	$ 0.19us-gaap_EarningsPerShareBasic	$ 0.25us-gaap_EarningsPerShareBasic	$ 0.25us-gaap_EarningsPerShareBasic	$ 0.16us-gaap_EarningsPerShareBasic	$ 0.94us-gaap_EarningsPerShareBasic	$ 0.84us-gaap_EarningsPerShareBasic	$ 0.76us-gaap_EarningsPerShareBasic
Income per share Diluted (in dollars per share)	$ 0.21us-gaap_EarningsPerShareDiluted	$ 0.28us-gaap_EarningsPerShareDiluted	$ 0.28us-gaap_EarningsPerShareDiluted	$ 0.18us-gaap_EarningsPerShareDiluted	$ 0.19us-gaap_EarningsPerShareDiluted	$ 0.25us-gaap_EarningsPerShareDiluted	$ 0.25us-gaap_EarningsPerShareDiluted	$ 0.16us-gaap_EarningsPerShareDiluted	$ 0.94us-gaap_EarningsPerShareDiluted	$ 0.84us-gaap_EarningsPerShareDiluted	$ 0.76us-gaap_EarningsPerShareDiluted

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

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