SIGNIFICANT ACCOUNTING POLICIES AND RECENT ACCOUNTING PRONOUNCEMENTS Prior Period Statements of Cash Flows Adjustments, Adoption of ASU 2014-09 (Details) - USD ($) $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2017	[1]	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net income	$ (16,898)		$ 16,506	$ 13,902	$ 14,099	$ 14,303	$ 12,809	$ 11,407	$ 10,379	$ 27,609	$ 48,898	$ 39,152
Deferred income taxes										15,838	(638)	(5,508)
Accounts receivable										(10,479)	751	4,144
Prepaid expenses and other assets										(17,646)	(2,524)	3,342
Taxes payable and accrued liabilities										$ 20,041	(1,124)	(75)
Increase (Decrease) in Deferred Income											0	0
As Reported
Net income											47,890	39,147
Deferred income taxes											(660)	(5,416)
Accounts receivable											650	4,131
Prepaid expenses and other assets											(2,499)	3,391
Taxes payable and accrued liabilities											(1,124)	(76)
Increase (Decrease) in Deferred Income											1,106	(122)
Adjustments for Accounting Standards Update 2014-09 [Member]
Net income											1,008	5
Deferred income taxes											22	(92)
Accounts receivable											101	13
Prepaid expenses and other assets											(25)	(49)
Taxes payable and accrued liabilities											0	1
Increase (Decrease) in Deferred Income											$ (1,106)	$ 122

[1] In December 2017 the U.S. government enacted comprehensive tax legislation commonly referred to as the Tax Cuts and Jobs Act (Refer to Note 11, Provision for Income Taxes, in the Notes to Consolidated Financial Statements).