Fair Value Measurements - Recurring (Details) - USD ($)	Dec. 31, 2019	Aug. 30, 2019	Jul. 31, 2019	Dec. 31, 2018	Jun. 01, 2015
Assets:
Pension plans	$ 17,099,000			$ 15,800,000
Senior Notes 6.25 Percent Due 2022
Liabilities:
Stated interest rate (as a percent)	6.25%	6.25%
Face value of debt instrument				375,000,000
Fair value of debt instrument				363,750,000
Senior Notes 7.00 Percent Due 2027 [Member]
Liabilities:
Stated interest rate (as a percent)	7.00%		7.00%
Face value of debt instrument	$ 400,000,000		$ 400,000,000
Fair value of debt instrument	$ 423,000,000
Senior Notes 7.00 Percent Due 2023 [Member]
Liabilities:
Stated interest rate (as a percent)	7.00%				7.00%
Face value of debt instrument	$ 300,000,000			300,000,000	$ 300,000,000
Fair value of debt instrument	309,000,000			294,000,000
Recurring basis | Exchange-Traded Derivatives
Assets:
Cash collateral netting	33,013,000			(68,851,000)
Recurring basis | Total estimated fair value
Assets:
Pension plans	17,099,000			15,800,000
Total assets	56,129,000			56,956,000
Recurring basis | Total estimated fair value | Forward Derivative Contracts
Assets:
Derivative assets	4,564,000			26,390,000
Liabilities:
Derivative liabilities	(12,698,000)			(4,494,000)
Recurring basis | Total estimated fair value | Exchange-Traded Derivatives
Assets:
Exchange-traded/cleared derivative instruments	34,466,000			14,766,000
Recurring basis | Total estimated fair value | Level 1
Assets:
Pension plans	17,099,000			15,800,000
Total pre-netting liabilities	18,552,000			99,417,000
Recurring basis | Total estimated fair value | Level 1 | Exchange-Traded Derivatives
Assets:
Exchange-traded/cleared derivative instruments	1,453,000			83,617,000
Recurring basis | Total estimated fair value | Level 2
Assets:
Total assets	4,002,000			25,504,000
Recurring basis | Total estimated fair value | Level 2 | Forward Derivative Contracts
Assets:
Derivative assets	4,002,000			25,504,000
Liabilities:
Derivative liabilities	(12,112,000)			(3,878,000)
Recurring basis | Total estimated fair value | Level 3
Assets:
Total assets	562,000			886,000
Recurring basis | Total estimated fair value | Level 3 | Forward Derivative Contracts
Assets:
Derivative assets	562,000			886,000
Liabilities:
Derivative liabilities	$ (586,000)			$ (616,000)