EMPLOYEE COMPENSATION PLANS (NARRATIVE) (DETAILS) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2011
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Company Match on US Defined Contribution Plans		$ 5,700	$ 5,100	$ 4,600
Shares Authorized under Current Equity Compensation Plan	4,000,000	4,000,000
Shares Available for Issuance under Current Equity Compensation Plan	1,100,000	1,100,000
Exercise of stock options, share		60,000
Equity-based compensation expense		$ 11,852	9,773	11,304
Total Tax Benefit Recognized for Equity-Based Compensation Expense		$ 6,800	5,000	6,700
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period Weighted Average Grant Date Fair Value		$ 29.56
Intrinsic Value of Non-Vested RSUs	$ 51,900	$ 51,900
Intrinsic Value of Options Exercised		194
Fair Value of Shares Vested				0
Proceeds from exercise of stock options		2,150	371	825
Recognized Tax Benefit from Exercise of Stock Options		0	200	1,000
Stock Options [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Intrinsic Value of Options Exercised		194,000	$ 400,000	$ 13,900
Stock Options [Member] | Key Employee1 [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Options Granted to Key Employees					150,000
Restricted Stock Units (RSUs) [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Unrecognized Compensation Expense	23,700	$ 23,700
Requisite Service Period		1 year 6 months
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period Weighted Average Grant Date Fair Value		$ 29.56	$ 26.60	$ 26.52
Intrinsic Value of RSUs Vested		$ 10,600	$ 10,800	$ 13,000
Restricted Stock Units (RSUs) [Member] | Executive Vice President [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Equity-based compensation expense		$ 400	100
Cost of Services
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Equity-based compensation expense	$ 4,100		3,100	2,900
Selling General And Administrative Expenses [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Equity-based compensation expense			$ 6,700	$ 8,400