Significant Accounting Policies - Regulatory Asset Disclosure (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Regulatory Assets [Line Items]
Current regulatory assets	$ 41,241	$ 45,781	$ 49,504
Non-current regulatory assets	333,917	356,608	345,352
Deferred Derivative Gain (Loss)
Regulatory Assets [Line Items]
Current regulatory assets	8,828	18,712	8,887
Non-current regulatory assets	3,016	4,649	3,926
Gas costs
Regulatory Assets [Line Items]
Current regulatory assets	461	154	1,851
Non-current regulatory assets	14	84	48
Environmental Restoration Costs
Regulatory Assets [Line Items]
Current regulatory assets	5,633	6,198	6,362
Non-current regulatory assets	63,464	72,128	63,339
Decoupling
Regulatory Assets [Line Items]
Current regulatory assets	11,990	11,227	15,663
Non-current regulatory assets	829	3,970	1,025
Income taxes
Regulatory Assets [Line Items]
Current regulatory assets	2,217	2,218	4,378
Non-current regulatory assets	19,267	19,991	36,591
Other
Regulatory Assets [Line Items]
Current regulatory assets	12,112	7,272	12,363
Non-current regulatory assets	9,295	15,579	10,137
Pension balancing
Regulatory Assets [Line Items]
Non-current regulatory assets	72,291	60,383	57,599
Pension and other postretirement benefit liabilities
Regulatory Assets [Line Items]
Non-current regulatory assets	$ 165,741	$ 179,824	$ 172,687