SUPPLEMENTAL CONDENSED CONSOLIDATING FINANCIAL INFORMATION - Condensed Consolidating Balance Sheet (Details) - USD ($) $ in Millions	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 27.0			$ 20.0
Trade accounts receivable, net	259.3			226.6
Inventories	53.9			51.2
Other receivables, net	13.3			18.4
Prepaid expenses and other	9.0			7.9
Intercompany receivables	0.0			0.0
Total current assets	362.5			324.1
Property, plant and equipment, net	676.3			680.2
Operating lease assets	69.3
Goodwill	239.5			239.3
Intangible assets, net	98.0			116.6
Investment in subsidiaries	0.0			0.0
Long-term intercompany receivables	0.0			0.0
Other assets	10.7			11.1
Total assets	1,456.3			1,371.3
Current liabilities:
Accounts payable	135.8			125.8
Accrued liabilities	90.3			96.3
Current maturities of long-term debt	35.6			30.8
Current operating lease liabilities	13.5
Intercompany payables	0.0			0.0
Total current liabilities	275.2			252.9
Long-term debt, net of current maturities	670.1			683.3
Long-term operating lease liabilities	58.6
Other long-term obligations and deferred credits	44.0			54.8
Deferred income taxes	44.9			43.1
Long-term intercompany payables	0.0			0.0
Total liabilities	1,092.8			1,034.1
Total shareholders' equity	337.8			312.4
Non-controlling interest	25.7			24.8
Total equity	363.5	$ 344.6	$ 335.4	337.2	$ 336.5	$ 317.8	$ 299.1	$ 302.1
Total liabilities and equity	1,456.3			1,371.3
Reportable Legal Entities | Parent
Current assets:
Cash and cash equivalents	0.0			0.0
Trade accounts receivable, net	0.0			0.0
Inventories	0.0			0.0
Other receivables, net	10.9			11.1
Prepaid expenses and other	0.0			0.0
Intercompany receivables	9.7			9.7
Total current assets	20.6			20.8
Property, plant and equipment, net	0.0			0.0
Operating lease assets	0.0
Goodwill	0.0			0.0
Intangible assets, net	0.0			0.0
Investment in subsidiaries	633.2			604.1
Long-term intercompany receivables	309.8			308.9
Other assets	0.0			0.0
Total assets	963.6			933.8
Current liabilities:
Accounts payable	0.0			0.0
Accrued liabilities	13.9			4.7
Current maturities of long-term debt	0.3			0.3
Current operating lease liabilities	0.0
Intercompany payables	0.0			0.0
Total current liabilities	14.2			5.0
Long-term debt, net of current maturities	611.6			615.5
Long-term operating lease liabilities	0.0
Other long-term obligations and deferred credits	0.0			0.9
Deferred income taxes	0.0			0.0
Long-term intercompany payables	0.0			0.0
Total liabilities	625.8			621.4
Total shareholders' equity	337.8			312.4
Non-controlling interest	0.0			0.0
Total equity	337.8			312.4
Total liabilities and equity	963.6			933.8
Reportable Legal Entities | Guarantor Subsidiaries
Current assets:
Cash and cash equivalents	17.8			10.8
Trade accounts receivable, net	239.9			219.7
Inventories	43.2			42.4
Other receivables, net	9.0			7.0
Prepaid expenses and other	8.4			7.1
Intercompany receivables	0.0			0.0
Total current assets	318.3			287.0
Property, plant and equipment, net	470.0			468.3
Operating lease assets	56.0
Goodwill	155.7			155.5
Intangible assets, net	93.7			111.8
Investment in subsidiaries	0.0			0.0
Long-term intercompany receivables	0.0			0.0
Other assets	9.0			10.8
Total assets	1,102.7			1,033.4
Current liabilities:
Accounts payable	131.7			122.4
Accrued liabilities	75.7			83.2
Current maturities of long-term debt	34.7			29.9
Current operating lease liabilities	11.7
Intercompany payables	0.0			0.0
Total current liabilities	253.8			235.5
Long-term debt, net of current maturities	56.9			67.6
Long-term operating lease liabilities	46.8
Other long-term obligations and deferred credits	39.0			51.0
Deferred income taxes	22.4			22.4
Long-term intercompany payables	192.8			188.7
Total liabilities	611.7			565.2
Total shareholders' equity	491.0			468.2
Non-controlling interest	0.0			0.0
Total equity	491.0			468.2
Total liabilities and equity	1,102.7			1,033.4
Reportable Legal Entities | Non-Guarantor Subsidiaries
Current assets:
Cash and cash equivalents	9.2			9.2
Trade accounts receivable, net	19.4			6.9
Inventories	10.7			8.8
Other receivables, net	0.5			0.3
Prepaid expenses and other	0.6			0.8
Intercompany receivables	0.3			0.3
Total current assets	40.7			26.3
Property, plant and equipment, net	206.3			211.9
Operating lease assets	13.3
Goodwill	83.8			83.8
Intangible assets, net	4.3			4.8
Investment in subsidiaries	0.0			0.0
Long-term intercompany receivables	1.1			1.1
Other assets	1.7			0.3
Total assets	351.2			328.2
Current liabilities:
Accounts payable	4.1			3.4
Accrued liabilities	7.8			8.4
Current maturities of long-term debt	0.6			0.6
Current operating lease liabilities	1.8
Intercompany payables	10.0			10.0
Total current liabilities	24.3			22.4
Long-term debt, net of current maturities	1.6			0.2
Long-term operating lease liabilities	11.8
Other long-term obligations and deferred credits	5.0			2.9
Deferred income taxes	22.5			20.7
Long-term intercompany payables	118.1			121.3
Total liabilities	183.3			167.5
Total shareholders' equity	142.2			135.9
Non-controlling interest	25.7			24.8
Total equity	167.9			160.7
Total liabilities and equity	351.2			328.2
Eliminations
Current assets:
Cash and cash equivalents	0.0			0.0
Trade accounts receivable, net	0.0			0.0
Inventories	0.0			0.0
Other receivables, net	(7.1)			0.0
Prepaid expenses and other	0.0			0.0
Intercompany receivables	(10.0)			(10.0)
Total current assets	(17.1)			(10.0)
Property, plant and equipment, net	0.0			0.0
Operating lease assets	0.0
Goodwill	0.0			0.0
Intangible assets, net	0.0			0.0
Investment in subsidiaries	(633.2)			(604.1)
Long-term intercompany receivables	(310.9)			(310.0)
Other assets	0.0			0.0
Total assets	(961.2)			(924.1)
Current liabilities:
Accounts payable	0.0			0.0
Accrued liabilities	(7.1)			0.0
Current maturities of long-term debt	0.0			0.0
Current operating lease liabilities	0.0
Intercompany payables	(10.0)			(10.0)
Total current liabilities	(17.1)			(10.0)
Long-term debt, net of current maturities	0.0			0.0
Long-term operating lease liabilities	0.0
Other long-term obligations and deferred credits	0.0			0.0
Deferred income taxes	0.0			0.0
Long-term intercompany payables	(310.9)			(310.0)
Total liabilities	(328.0)			(320.0)
Total shareholders' equity	(633.2)			(604.1)
Non-controlling interest	0.0			0.0
Total equity	(633.2)			(604.1)
Total liabilities and equity	$ (961.2)			$ (924.1)