Income Tax (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Components of Deferred Tax Assets and Liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	$ 428,642	$ 408,342
Deferred Tax Liabilities, Regulatory Assets	43,048	47,427
Deferred Tax Liabilities, Regulatory Assets and Liabilities	48,291	46,400
Deferred Tax Liabilities Non Regulated Liabilities	51,446	49,683
Deferred Tax Liabilities, Net	571,427	551,852
Deferred Tax Assets, Tax Deferred Expense, Pension and Postretirement Obligations	4,493	4,666
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax	9,853	16,699
Deferred Tax Assets, Tax Credit Carryforwards, Other	0	514
Deferred Tax Assets, Net	14,346	21,879
Deferred Tax Assets (Liabilities), Net	557,081	529,973
Accumulated Deferred Investment Tax Credit	4	48
Deferred Tax Liabilities, Other	557,085	530,021
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate	34,863	31,310	$ 35,117
Income Tax Reconciliation, State and Local Income Taxes, net of federal tax benefit	4,582	4,195	4,666
Income Tax Reconciliation, Tax Credits, Investment	(41)	(118)	(201)
Income Tax Reconciliation, Differences Regulatory Commission	2,357	2,357	2,357
Income Tax Reconciliation, Gains on company and trust-owned life insurance	(594)	(766)	(689)
Income Tax Reconciliation, Other Adjustments	(453)	(1,225)	393
Income Tax Expense (Benefit)	$ 40,714	$ 35,753	$ 41,643
Effective Income Tax Rate	40.90%	40.00%	41.50%
Income Tax Disclosure [Abstract]
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax	$ 9,853	$ 16,699
Income Tax expense decrease after deferred depletion is realized	900
Tax Provision Current And Deferred [Abstract]
Current Federal Tax Expense (Benefit)	7,402	10,558	$ 14,823
Current State and Local Tax Expense (Benefit)	2,042	61	24
Current Income Tax Expense (Benefit)	9,444	10,619	14,847
Deferred Federal Income Tax Expense (Benefit)	26,219	18,729	18,635
Deferred State and Local Income Tax Expense (Benefit)	5,051	6,405	8,161
Deferred Income Tax Expense (Benefit)	31,270	25,134	26,796
Income Tax Expense (Benefit)	40,714	35,753	41,643
Tax Provision Regulated And Nonutility [Abstract]
Regulated Current Tax Expense Benefit	10,300	15,890	24,317
Regulated Deferred And Local Tax Expense Benefit	28,749	20,834	19,518
Regulated Investments And Credits Tax Expense Benefit	(41)	(118)	(201)
Total Tax Provision Related To Regulated Utility	39,008	36,606	43,634
Non Utility Current Tax Expense Benefit	(856)	(5,271)	(9,470)
Non Utility Deferred And Local Tax Expense Benefit	2,562	4,418	7,479
Total Tax Provision Related To Non Utility	1,706	(853)	(1,991)
Income Tax Expense (Benefit)	$ 40,714	$ 35,753	$ 41,643