BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Effect of Adoption of Topic 606) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Assets
Unbilled revenues	$ 104,652				$ 86,500				$ 104,652	$ 86,500		$ 86,422
Deferred tax assets												24,801
Liabilities
Accrued expenses and other current liabilities	127,937								127,937			89,104
Other noncurrent liabilities	17,661				9,435				17,661	9,435
Stockholders’ equity
Retained earnings	759,533				518,820				759,533	518,820		519,277
Income Statement
Revenues	504,931	$ 468,186	$ 445,647	$ 424,148	399,297	$ 377,523	$ 348,977	$ 324,651	1,842,912	1,450,448	$ 1,160,132
Income from operations	78,270	64,560	54,237	48,697	52,050	49,248	40,682	30,966	245,764	172,946	133,696
Provision for income taxes	18,803	369	6,864	(16,519)	82,951	7,953	5,687	4,954	9,517	101,545	27,200
Net income	59,965	$ 65,618	$ 50,255	$ 64,418	(30,874)	$ 42,634	$ 37,359	$ 23,641	240,256	72,760	$ 99,266
Balance Under Revenue Guidance in Effect Before Topic 606
Assets
Unbilled revenues					86,500					86,500
Deferred tax assets					24,974					24,974
Liabilities
Accrued expenses and other current liabilities	127,690				89,812				127,690	89,812
Other noncurrent liabilities	17,716								17,716
Stockholders’ equity
Retained earnings	759,725				$ 518,820				759,725	$ 518,820
Income Statement
Revenues									1,843,159
Income from operations									246,011
Provision for income taxes									9,572
Net income									240,448
Topic 606 | Adjustments
Assets
Unbilled revenues												(78)
Deferred tax assets												(173)
Liabilities
Accrued expenses and other current liabilities	247								247			(708)
Other noncurrent liabilities	(55)								(55)
Stockholders’ equity
Retained earnings	$ (192)								(192)			$ 457
Income Statement
Revenues									(247)
Income from operations									(247)
Provision for income taxes									(55)
Net income									$ (192)