Significant Accounting Policies - Regulatory Asset Disclosure (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Regulatory Assets [Line Items]
Regulatory Assets, Current	$ 45,900	$ 45,781	$ 34,874
Non-current regulatory assets	343,037	356,608	349,057
Deferred Derivative Gain (Loss) [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	17,569	18,712	1,580
Non-current regulatory assets	2,355	4,649	2,546
Gas costs
Regulatory Assets [Line Items]
Regulatory Assets, Current	591	154	2,757
Non-current regulatory assets	49	84	114
Environmental Restoration Costs
Regulatory Assets [Line Items]
Regulatory Assets, Current	5,818	6,198	7,574
Non-current regulatory assets	66,730	72,128	62,227
Decoupling
Regulatory Assets [Line Items]
Regulatory Assets, Current	9,578	11,227	10,087
Non-current regulatory assets	2,663	3,970	2,803
Income taxes
Regulatory Assets [Line Items]
Regulatory Assets, Current	2,218	2,218	4,378
Non-current regulatory assets	19,267	19,991	36,591
Other
Regulatory Assets [Line Items]
Regulatory Assets, Current	10,126	7,272	8,498
Non-current regulatory assets	12,528	15,579	12,085
Pension balancing
Regulatory Assets [Line Items]
Non-current regulatory assets	63,940	60,383	53,105
Pension and other postretirement benefit liabilities
Regulatory Assets [Line Items]
Non-current regulatory assets	$ 175,505	$ 179,824	$ 179,586