BUSINESS COMBINATIONS - Motive Merger (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
	Jun. 02, 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, 2017	Sep. 30, 2016
Business Combinations
Payment to acquire business, net										$ 47,886	$ 70,416
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
Fair value of assets acquired
Goodwill		64,777				51,705				64,777	51,705	$ 4,718
Fair value of liabilities assumed
Contingent earnout liability		11,160				14,879				$ 11,160	14,879
Estimated useful lives (in years)										15 years
MOTIVE Merger
Business Combinations
Operating revenue										$ 12,900	3,300
Net loss										20,100	2,200
Future potential earnout payments	$ 25,000
Cash placed in escrow	$ 9,400
Percentage of escrow to be released to seller on each of twelve and eighteenth month anniversaries	50.00%
Purchase Price
Cash consideration	$ 74,275
Long-term contingent earnout liability (Other noncurrent liabilities)	14,509
Total cash consideration given	88,784
Fair value of assets acquired
Current assets	4,425
Property, plant and equipment	300
Intangible assets, net	51,000
Goodwill	46,987
Total assets acquired	102,712
Fair value of liabilities assumed
Current liabilities	25
Deferred income taxes	13,903
Total liabilities acquired	13,928
Fair value of total assets acquired and liabilities assumed	$ 88,784
Contingent consideration paid										10,600
Change in fair value of contingent consideration										6,900	400
Estimated useful lives (in years)	15 years
MOTIVE Merger | (Level 3)
Fair value of liabilities assumed
Contingent earnout liability		$ 11,200				14,900				$ 11,200	14,900
MOTIVE Merger | General and Administrative
Business Combinations
Business acquisition transaction cost						$ 3,200					$ 3,200