SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Summary of Impact of Adopting ASC 606 on Consolidated Financials) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Statements of Operations
Revenue	$ 159,255	$ 158,149	$ 561,496	$ 565,720
Income tax expense	4,592	6,365	11,738	19,638
Net income	15,994	15,027	44,933	46,946
ASSETS
Prepaid income taxes	693		693		$ 6,441
Prepaid expenses and other current assets	8,237		8,237		11,758
LIABILITIES
Deferred revenue	9,287		9,287		7,991
Other accrued expenses and current liabilities	5,883		5,883		6,068
EQUITY
Retained earnings	106,436		106,436		$ 83,035
Statement of Cash Flows
Net income	15,994	$ 15,027	44,933	46,946
Changes in operating assets and liabilities:
Prepaid expenses and other assets			3,602	2,945
Deferred revenue			2,633	1,891
Income taxes			5,794	(2,032)
Other accrued expenses and liabilities			(811)	$ (586)
Adjustments [Member] | ASC 606 [Member]
Statements of Operations
Revenue	111		319
Income tax expense	25		72
Net income	86		247
ASSETS
Prepaid income taxes	404		404
Prepaid expenses and other current assets	(181)		(181)
LIABILITIES
Deferred revenue	(1,676)		(1,676)
Other accrued expenses and current liabilities	71		71
EQUITY
Retained earnings	1,382		1,382
Statement of Cash Flows
Net income	86		247
Changes in operating assets and liabilities:
Prepaid expenses and other assets			(51)
Deferred revenue			(339)
Income taxes			72
Other accrued expenses and liabilities			71
Amounts without Adoption of ASC 606 [Member] | ASC 606 [Member]
Statements of Operations
Revenue	159,144		561,177
Income tax expense	4,567		11,666
Net income	15,908		44,686
ASSETS
Prepaid income taxes	1,097		1,097
Prepaid expenses and other current assets	8,056		8,056
LIABILITIES
Deferred revenue	10,963		10,963
Other accrued expenses and current liabilities	5,812		5,812
EQUITY
Retained earnings	105,054		105,054
Statement of Cash Flows
Net income	$ 15,908		44,686
Changes in operating assets and liabilities:
Prepaid expenses and other assets			3,653
Deferred revenue			2,972
Income taxes			5,722
Other accrued expenses and liabilities			$ (882)