Fair Value Measurements - Recurring (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Senior Notes 7.00 Percent Due 2027 [Member]
Liabilities:
Stated interest rate (as a percent)	7.00%
Face value of debt instrument	$ 400,000	$ 400,000
Fair value of debt instrument	$ 371,000	$ 423,000
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	$ 300,000
Fair value of debt instrument	285,000	309,000
Recurring basis
Assets:
Cash collateral netting	38,591	33,013
Recurring basis | Exchange-Traded Derivatives
Assets:
Cash collateral netting	38,591	33,013
Recurring basis | Total estimated fair value
Assets:
Pension plans	16,668	17,099
Total assets	97,535	56,129
Recurring basis | Total estimated fair value | Forward Derivative Contracts
Assets:
Derivative assets	48,571	4,564
Liabilities:
Derivative liabilities	(8,089)	(12,698)
Recurring basis | Total estimated fair value | Exchange-Traded Derivatives
Assets:
Exchange-traded/cleared derivative instruments	32,296	34,466
Recurring basis | Total estimated fair value | Level 1
Assets:
Pension plans	16,668	17,099
Total assets	10,373	18,552
Recurring basis | Total estimated fair value | Level 1 | Exchange-Traded Derivatives
Assets:
Exchange-traded/cleared derivative instruments	(6,295)	1,453
Recurring basis | Total estimated fair value | Level 2
Assets:
Total assets	47,852	4,002
Recurring basis | Total estimated fair value | Level 2 | Forward Derivative Contracts
Assets:
Derivative assets	47,852	4,002
Liabilities:
Derivative liabilities	(7,746)	(12,112)
Recurring basis | Total estimated fair value | Level 3
Assets:
Total assets	719	562
Recurring basis | Total estimated fair value | Level 3 | Forward Derivative Contracts
Assets:
Derivative assets	719	562
Liabilities:
Derivative liabilities	$ (343)	$ (586)