CONDENSED CONSOLIDATING FINANCIAL INFORMATION - Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities:
Net cash (used in) provided by operating activities	$ 84,990	$ 74,997
Cash flows from investing activities:
Purchases of property, plant and equipment	(42,389)	(53,394)
Proceeds from sale of assets	2,187	865
Distributions from investments	426	329
Net cash used in investing activities	(39,776)	(52,200)
Cash flows from financing activities:
Proceeds from issuance of long-term debt	10,000	51,000
Payment of finance lease obligations	(2,674)	(3,507)
Payment on long-term debt	(46,588)	(45,588)
Repurchase of senior notes	(4,208)
Dividends on common stock		(27,577)
Net cash used in financing activities	(43,470)	(25,672)
Change in cash and cash equivalents	1,744	(2,875)
Cash and cash equivalents at beginning of period	12,395	9,599
Cash and cash equivalents at end of period	14,139	6,724
Parent
Cash flows from operating activities:
Net cash (used in) provided by operating activities	(354)	(178)
Cash flows from financing activities:
Dividends on common stock		(27,577)
Transactions with affiliates, net	354	27,755
Net cash used in financing activities	354	178
Subsidiary Issuer
Cash flows from operating activities:
Net cash (used in) provided by operating activities	(8,877)	(9,043)
Cash flows from financing activities:
Proceeds from issuance of long-term debt	10,000	51,000
Payment on long-term debt	(46,588)	(45,588)
Repurchase of senior notes	(4,208)
Transactions with affiliates, net	51,422	(1,206)
Net cash used in financing activities	10,626	4,206
Change in cash and cash equivalents	1,749	(4,837)
Cash and cash equivalents at beginning of period	12,387	9,616
Cash and cash equivalents at end of period	14,136	4,779
Guarantors
Cash flows from operating activities:
Net cash (used in) provided by operating activities	88,395	77,551
Cash flows from investing activities:
Purchases of property, plant and equipment	(40,923)	(51,617)
Proceeds from sale of assets	2,184	863
Distributions from investments	426	329
Net cash used in investing activities	(38,313)	(50,425)
Cash flows from financing activities:
Payment of finance lease obligations	(2,641)	(3,471)
Transactions with affiliates, net	(47,446)	(21,693)
Net cash used in financing activities	(50,087)	(25,164)
Change in cash and cash equivalents	(5)	1,962
Cash and cash equivalents at beginning of period	8	(18)
Cash and cash equivalents at end of period	3	1,944
Non-Guarantors
Cash flows from operating activities:
Net cash (used in) provided by operating activities	5,826	6,667
Cash flows from investing activities:
Purchases of property, plant and equipment	(1,466)	(1,777)
Proceeds from sale of assets	3	2
Net cash used in investing activities	(1,463)	(1,775)
Cash flows from financing activities:
Payment of finance lease obligations	(33)	(36)
Transactions with affiliates, net	(4,330)	(4,856)
Net cash used in financing activities	$ (4,363)	(4,892)
Cash and cash equivalents at beginning of period		1
Cash and cash equivalents at end of period		$ 1