Fair Value Measurements - Recurring (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Jun. 01, 2015	Jun. 19, 2014
Assets:
Pension plans	$ 15,800,000	$ 17,581,000
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	375,000,000		$ 375,000,000
Fair value of debt instrument	$ 363,750,000	383,906,000
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	294,000,000	308,250,000
Recurring basis | Exchange-Traded Derivatives
Assets:
Cash collateral netting	(68,851,000)	48,281,000
Recurring basis | Total estimated fair value
Assets:
Pension plans	15,800,000	17,581,000
Total assets	56,956,000	31,102,000
Liabilities:
Total liabilities		(13,842,000)
Recurring basis | Total estimated fair value | Forward Derivative Contracts
Assets:
Derivative assets	25,069,000	3,840,000
Liabilities:
Derivative liabilities	(4,494,000)	(13,708,000)
Recurring basis | Total estimated fair value | Swap And Options [Member]
Assets:
Derivative assets	1,321,000
Recurring basis | Total estimated fair value | Interest Rate Swaps
Liabilities:
Derivative liabilities		(134,000)
Recurring basis | Total estimated fair value | Exchange-Traded Derivatives
Assets:
Exchange-traded/cleared derivative instruments	14,766,000	9,681,000
Recurring basis | Total estimated fair value | Level 1
Assets:
Pension plans	15,800,000	17,581,000
Total pre-netting liabilities	99,417,000	(21,019,000)
Recurring basis | Total estimated fair value | Level 1 | Exchange-Traded Derivatives
Assets:
Exchange-traded/cleared derivative instruments	83,617,000	(38,600,000)
Recurring basis | Total estimated fair value | Level 2
Assets:
Total assets	25,504,000	3,207,000
Liabilities:
Total liabilities		(12,805,000)
Recurring basis | Total estimated fair value | Level 2 | Forward Derivative Contracts
Assets:
Derivative assets	24,183,000	3,207,000
Liabilities:
Derivative liabilities	(3,878,000)	(12,671,000)
Recurring basis | Total estimated fair value | Level 2 | Swap And Options [Member]
Assets:
Derivative assets	1,321,000
Recurring basis | Total estimated fair value | Level 2 | Interest Rate Swaps
Liabilities:
Derivative liabilities		(134,000)
Recurring basis | Total estimated fair value | Level 3
Assets:
Total assets	886,000	633,000
Liabilities:
Total liabilities		(1,037,000)
Recurring basis | Total estimated fair value | Level 3 | Forward Derivative Contracts
Assets:
Derivative assets	886,000	633,000
Liabilities:
Derivative liabilities	$ (616,000)	$ (1,037,000)