Significant Accounting Policies - Property, Plant and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Depreciation, depletion and amortization	$ 501,821	$ 508,463	$ 493,423
Building and Improvements \| Minimum
Property, Plant and Equipment [Line Items]
Useful life of property, plant and equipment	10 years
Building and Improvements \| Maximum
Property, Plant and Equipment [Line Items]
Useful life of property, plant and equipment	45 years
Machinery and Equipment \| Minimum
Property, Plant and Equipment [Line Items]
Useful life of property, plant and equipment	3 years
Machinery and Equipment \| Maximum
Property, Plant and Equipment [Line Items]
Useful life of property, plant and equipment	25 years
Gathering and Transmission \| Minimum
Property, Plant and Equipment [Line Items]
Useful life of property, plant and equipment	30 years
Gathering and Transmission \| Maximum
Property, Plant and Equipment [Line Items]
Useful life of property, plant and equipment	40 years
Gas Properties
Property, Plant and Equipment [Line Items]
Depreciation, depletion and amortization	$ 400,758	$ 423,488	$ 412,588
Purchased Software
Property, Plant and Equipment [Line Items]
Useful life of property, plant and equipment	7 years