Income Taxes (Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Income Tax Contingency [Line Items]
Full valuation allowance	$ 4,200
Deferred tax assets:
Total deferred tax assets, Current	0	$ 25,533
Total deferred tax assets, Noncurrent	81,804	70,320
Valuation allowance, Current	0	(1,122)
Valuation allowance, Noncurrent	(4,235)	(2,467)
Total deferred tax assets, net, Current	0	24,411
Total deferred tax assets, net, Noncurrent	77,569	67,853
Deferred tax liabilities:
Total deferred tax liabilities, current	0	103,575
Total deferred tax liabilities, noncurrent	629,938	1,068,339
Deferred tax liabilities, net, current	0	(79,164)
Deferred tax liabilities, net, noncurrent	(552,369)	(1,000,486)
Oil and Gas Operations
Income Tax Contingency [Line Items]
State operating loss carryforwards and other tax carryforwards portion about to expire	191,300
Deferred tax assets, Current
Deferred tax assets:
Minimum tax credit	0	0
Allowance for doubtful accounts	0	244
Insurance and other accruals	0	2,537
Compensation accruals	0	11,355
Pension and other costs	0	0
Other comprehensive income	0	10,732
State net operating losses and other carryforwards	0	0
Other	0	665
Deferred tax assets, Noncurrent
Income Tax Contingency [Line Items]
State net operating loss carryforwards, subject to expiration	8,400
Deferred tax assets:
Minimum tax credit	44,862	46,338
Allowance for doubtful accounts	253	0
Insurance and other accruals	2,807	0
Compensation accruals	11,650	0
Pension and other costs	8,693	7,009
Other comprehensive income	0	1,581
State net operating losses and other carryforwards	12,577	15,392
Other	962	0
Deferred tax liabilities, Current
Deferred tax liabilities:
Depreciation and basis differences	0	0
Derivative instruments	0	102,691
Other comprehensive income	0	0
Other	0	884
Deferred tax liabilities, Noncurrent
Deferred tax liabilities:
Depreciation and basis differences	620,629	1,057,430
Derivative instruments	2,838	0
Other comprehensive income	141	0
Other	6,330	$ 10,909
New Accounting Pronouncement, Early Adoption, Effect
Income Tax Contingency [Line Items]
Amount of reclassified current deferred tax asset	(14,500)
Deferred tax liabilities:
Deferred tax liabilities, net, noncurrent	$ (14,500)