Income Taxes Net Deferred Tax assets/Liabilities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Postretirement Benefits	$ 260,959,000	$ 257,604,000
Deferred Tax Assets, Operating Loss Carryforwards, Domestic	144,450,000	165,951,000
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Asset Retirement Obligations	39,860,000	63,399,000
Deferred Tax Assets, Property, Plant and Equipment	2,925,000	4,404,000
Deferred Tax Assets, Equity Method Investments	0	45,746,000
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax	219,872,000	143,122,000
deferred tax assets - gas well closing	68,585,000	79,246,000
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits	43,997,000	44,830,000
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Other	30,758,000	31,544,000
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Pensions	42,393,000	30,177,000
Deferred Tax Assets, Operating Loss Carryforwards	74,310,000	76,171,000
Deferred Tax Assets, Hedging Transactions	72,105,000	0
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Accrued Liabilities	29,532,000	44,317,000
Deferred Tax Assets, Tax Credit Carryforwards, Foreign	39,850,000	39,850,000
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals	0	14,122,000
Deferred Tax Assets, Other	66,724,000	65,427,000
Deferred Tax Assets, Gross	1,136,320,000	1,105,910,000
Deferred Tax Assets, Valuation Allowance	(282,778,000)	(78,306,000)
Deferred Tax Assets	853,542,000	1,027,604,000
Deferred Tax Liabilities, Property, Plant and Equipment	(782,710,000)	(946,778,000)
Deferred Tax Liabilities, Investment in Noncontrolled Affiliates	(40,200,000)	0
Deferred Tax Liabilities, Derivatives	0	(105,864,000)
Deferred Tax Liabilities, Leasing Arrangements	(22,326,000)	(44,921,000)
Deferred Tax Liabilities, Other	(4,016,000)	(4,670,000)
Deferred Tax Liabilities, Gross	(849,252,000)	(1,102,233,000)
Deferred Tax Liabilities, Net	4,290,000	(74,629,000)
Deferred Federal Income Tax Expense (Benefit)	74,310	76,171
Deferred Tax Assets, Valuation Allowance, Noncurrent	60,488,000	42,983,000
Valuation Allowance, Deferred Tax Asset, Change in Amount	$ 10,591,000	$ 9,420,000