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BUSINESS COMBINATIONS - MagVAR (Details) - USD ($) $ in Thousands		3 Months Ended								9 Months Ended	12 Months Ended
	Dec. 08, 2017	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Business Combinations
Operating revenue		$ 696,825	$ 648,872	$ 577,484	$ 564,087	$ 532,304	$ 498,564	$ 405,283	$ 368,590		$ 2,487,268	$ 1,804,741	$ 1,624,232
Net loss		(2,453)	$ 8,008	$ 11,879	$ (500,106)	22,532	$ 21,799	$ 48,818	$ 35,063		$ (482,672)	128,212	56,828
Estimated useful lives (in years)											15 years
Fair value of assets acquired
Goodwill		64,777				$ 51,705				$ 64,777	$ 64,777	51,705	$ 4,718
Minimum \| Restricted stock
Business Combinations
Vesting period											3 years
Maximum \| Restricted stock
Business Combinations
Vesting period											6 years
MagVAR Acquisition
Business Combinations
Aggregate cash consideration net of closing adjustments	$ 47,900
Period escrow to be released to seller	12 months
Operating revenue											$ 11,600
Net loss											3,000
Purchase Price
Cash consideration	$ 48,485
Fair value of assets acquired
Current assets	2,286
Property, plant and equipment	13
Intangible assets, net	28,700
Goodwill	17,791
Total assets acquired	48,790
Fair value of liabilities assumed
Current liabilities	305
Fair value of total assets acquired and liabilities assumed	48,485
Pro forma financial information:
After-tax transaction costs excluded from pro forma earnings										500
Revenues											2,490,955	1,814,215
Net income (loss)											480,423	$ (126,355)
MagVAR Acquisition \| Prepaid expenses and other
Business Combinations
Restricted cash	$ 6,000
MagVAR Acquisition \| Restricted stock
Business Combinations
Restricted stock awards issued	213,904
Grant date fair value of restricted stock	$ 13,100
Vesting period	3 years
MagVAR Acquisition \| General and Administrative
Business Combinations
Business acquisition transaction cost		$ 1,200								$ 1,200	$ 1,200
MagVAR Acquisition \| Minimum
Business Combinations
Estimated useful lives (in years)	5 years
MagVAR Acquisition \| Maximum
Business Combinations
Estimated useful lives (in years)	20 years