Business Combinations - Assets Acquired & Liabilities Assumed (Details 2) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Goodwill	$ 681,358,000	$ 664,922,000	$ 631,791,000
Measurement Period Adjustments
Goodwill	(6,669,000)	0
2019 Acquisition
Business Acquisition [Line Items]
Total current assets	15,800,000
Property, plant and equipment	16,700,000
Goodwill	23,100,000
Other intangible assets, net	6,700,000
Deferred charges and other assets - other	500,000
Current liabilities	5,900,000
Total deferred credits and other liabilities	100,000
2018 Acquisitions
Business Acquisition [Line Items]
Receivables, net	18,984,000	18,984,000
Inventories	10,101,000	10,329,000
Other current assets	501,000	515,000
Total current assets	29,586,000	29,828,000
Property, plant and equipment	138,435,000	131,766,000
Goodwill	26,462,000	33,131,000
Other intangible assets, net	8,227,000	8,227,000
Deferred charges and other assets - other	927,000	927,000
Total deferred charges and other assets	35,616,000	42,285,000
Total assets acquired	203,637,000	203,879,000
Current liabilities	10,880,000	11,122,000
Asset retirement obligation	914,000	914,000
Deferred income taxes	5,565,000	5,565,000
Total deferred credits and other liabilities	6,479,000	6,479,000
Total liabilities assumed	17,359,000	17,601,000
Total consideration (fair value)	186,278,000	$ 186,278,000
Measurement Period Adjustments
Inventories	(228,000)
Other current assets	(14,000)
Total current assets	(242,000)
Property, plant and equipment	6,669,000
Goodwill	(6,669,000)
Total deferred charges and other assets	(6,669,000)
Total assets acquired	(242,000)
Current liabilities	(242,000)
Total liabilities assumed	$ (242,000)