SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details 8) - 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
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Change in restricted cash										$ 0	$ 0
Net cash used in investing activities									$ (273,082)	(206,571)	(230,905)
Change in cash, cash equivalents, and restricted cash									(47,585)	69,365	16,589
Cash, cash equivalents, and restricted cash at beginning of year				$ 95,300				$ 25,935	95,300	25,935	9,346
Cash, cash equivalents, and restricted cash at end of year	$ 47,715				$ 95,300				47,715	95,300	25,935
Administrative and general									100,781	93,326	87,601
Income taxes									18,589	35,279	29,235
Total operating expenses									587,656	569,030	526,734
Net operating income	26,536	$ 47,329	$ 26,839	9,836	23,077	$ 43,902	$ 28,111	11,993	110,540	107,083	82,636
Other components of net periodic benefit cost									(9,308)	(9,588)	(10,873)
Income tax benefit (expense) on other income and expenses									2,717	(1,548)	2,419
Net other loss									(7,102)	(215)	(3,460)
Estimated total right-of-use assets and total lease liabilities	$ 13,800								13,800
As Adjusted
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Administrative and general										93,326
Income taxes										31,815
Total operating expenses										565,566
Net operating income										101,324
Other components of net periodic benefit cost										(9,588)
Income tax benefit (expense) on other income and expenses										(1,548)
Net other loss										(215)
As Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Change in restricted cash										(81)	66
Net cash used in investing activities										(206,652)	(230,839)
Change in cash, cash equivalents, and restricted cash										69,284	16,655
Cash, cash equivalents, and restricted cash at beginning of year				94,776				25,492	94,776	25,492	8,837
Cash, cash equivalents, and restricted cash at end of year					94,776					94,776	25,492
Administrative and general										102,914	98,474
Income taxes										28,928	24,804
Total operating expenses										572,267	533,176
Net operating income		$ 45,548	$ 25,056	8,053	21,638	$ 42,462	$ 26,671	10,553		94,623	76,194
Other components of net periodic benefit cost										0	0
Income tax benefit (expense) on other income and expenses										(4,435)	(2,012)
Net other loss										6,486	2,982
Adjustments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Income taxes										2,887
Total operating expenses										(6,701)
Net operating income										6,701
Accounting Standards Update 2016-18 | Adjustments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Change in restricted cash										81	(66)
Net cash used in investing activities										81	(66)
Change in cash, cash equivalents, and restricted cash										81	(66)
Cash, cash equivalents, and restricted cash at beginning of year				$ 524				$ 443	$ 524	443	509
Cash, cash equivalents, and restricted cash at end of year					$ 524					524	443
Accounting Standards Update 2017-07 | Adjustments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Administrative and general										(9,588)	(10,873)
Income taxes										2,887	4,431
Total operating expenses										(6,701)	(6,442)
Net operating income										6,701	6,442
Other components of net periodic benefit cost										(9,588)	(10,873)
Income tax benefit (expense) on other income and expenses										2,887	4,431
Net other loss										$ (6,701)	$ (6,442)