Income Tax (Details) - USD ($)		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 21, 2017	Dec. 31, 2016	Dec. 31, 2015
Regulatory Assets [Line Items]
Deferred Tax Liabilities, Regulatory Assets	$ 22,209,000	$ 22,209,000		$ 43,048,000
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax	66,000	66,000		9,853,000
Deferred Tax Assets, Regulatory Assets and Liabilities	56,470,000	56,470,000		0
Effect of TCJA		(21,429,000)		0	$ 0
Income Tax expense decrease after deferred depletion is realized					900,000
Deferred Tax Liabilities, Property, Plant and Equipment	296,114,000	296,114,000		428,642,000
Deferred Tax Liabilities, Regulatory Assets and Liabilities	29,114,000	29,114,000		48,291,000
Deferred Tax Liabilities Non Regulated Liabilities	933,000	933,000		51,446,000
Deferred Tax Liabilities, Net	348,370,000	348,370,000		571,427,000
Deferred Tax Assets Non Regulated Assets	17,796,000	17,796,000		0
Deferred Tax Assets, Tax Deferred Expense, Pension and Postretirement Obligations	3,512,000	3,512,000		4,493,000
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax	66,000	66,000		9,853,000
Deferred Tax Assets, Net	77,844,000	77,844,000		14,346,000
Deferred Tax Assets, Net	270,526,000	270,526,000		557,081,000
Accumulated Deferred Investment Tax Credit	0	0		4,000
Deferred Tax Liabilities, Other	$ 270,526,000	270,526,000		557,085,000
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%		35.00%
Regulatory Liability, Noncurrent	$ 586,093,000	586,093,000		349,319,000
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate		(30,233,000)		34,863,000	31,310,000
Income Tax Reconciliation, State and Local Income Taxes, net of federal tax benefit		(5,784,000)		4,582,000	4,195,000
Income Tax Reconciliation, Tax Credits, Investment		(4,000)		(41,000)	(118,000)
Income Tax Reconciliation, Differences Regulatory Commission		2,357,000		2,357,000	2,357,000
Income Tax Reconciliation, Gains on company and trust-owned life insurance		(872,000)		(594,000)	(766,000)
Deferred Tax Rate Differential, Post TCJA		26,947,000		0	0
Income Tax Reconciliation, Other Adjustments		(1,739,000)		(453,000)	(1,225,000)
Income Tax Expense (Benefit)		$ (30,757,000)		$ 40,714,000	$ 35,753,000
Effective Income Tax Rate		35.60%		40.90%	40.00%
Tax Provision Current And Deferred [Abstract]
Current Federal Tax Expense (Benefit)		$ 16,403,000		$ 7,402,000	$ 10,558,000
Current State and Local Tax Expense (Benefit)		4,892,000		2,042,000	61,000
Current Income Tax Expense (Benefit)		21,295,000		9,444,000	10,619,000
Deferred Federal Income Tax Expense (Benefit)		(41,134,000)		26,219,000	18,729,000
Deferred State and Local Income Tax Expense (Benefit)		(10,918,000)		5,051,000	6,405,000
Deferred Income Tax Expense (Benefit)		(52,052,000)		31,270,000	25,134,000
Income Tax Expense (Benefit)		(30,757,000)		40,714,000	35,753,000
Tax Provision Regulated And Nonutility [Abstract]
Regulated Current Tax Expense Benefit		21,453,000		10,300,000	15,890,000
Regulated Deferred And Local Tax Expense Benefit		19,479,000		28,749,000	20,834,000
Regulated Investments And Credits Tax Expense Benefit		(4,000)		(41,000)	(118,000)
Total Tax Provision Related To Regulated Utility		40,928,000		39,008,000	36,606,000
Non Utility Current Tax Expense Benefit		(158,000)		(856,000)	(5,271,000)
Non Utility Deferred And Local Tax Expense Benefit		(71,527,000)		2,562,000	4,418,000
Total Tax Provision Related To Non Utility		(71,685,000)		1,706,000	(853,000)
Income Tax Expense (Benefit)		(30,757,000)		40,714,000	$ 35,753,000
Other Regulatory Assets (Liabilities) [Member]
Regulatory Assets [Line Items]
Deferred Tax Liabilities, Regulatory Assets	21,300,000	21,300,000
Deferred Tax Liabilities AFUDC [Member]
Regulatory Assets [Line Items]
Deferred Tax Liabilities, Regulatory Assets	0.9	0.9
TCJA Revaluation [Member]
Regulatory Assets [Line Items]
Deferred Tax Assets, Regulatory Assets and Liabilities	18,200,000	18,200,000
Effect of TCJA		196,400,000.0
Deferred Income Tax Charge [Member]
Regulatory Assets [Line Items]
Regulatory Liability, Noncurrent	$ 213,306,000	$ 213,306,000		$ 0