Significant Accounting Policies - Regulatory Asset Disclosure (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Regulatory Assets [Line Items]
Current regulatory assets	$ 41,092	$ 45,781	$ 37,504
Non-current regulatory assets	339,177	356,608	348,284
Deferred Derivative Gain (Loss)
Regulatory Assets [Line Items]
Current regulatory assets	11,744	18,712	4,625
Non-current regulatory assets	3,913	4,649	3,466
Gas costs
Regulatory Assets [Line Items]
Current regulatory assets	273	154	859
Non-current regulatory assets	28	84	87
Environmental Restoration Costs
Regulatory Assets [Line Items]
Current regulatory assets	5,594	6,198	6,724
Non-current regulatory assets	65,156	72,128	64,008
Decoupling
Regulatory Assets [Line Items]
Current regulatory assets	10,232	11,227	12,136
Non-current regulatory assets	1,636	3,970	1,993
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	11,032	7,272	8,782
Non-current regulatory assets	10,464	15,579	10,645
Pension balancing
Regulatory Assets [Line Items]
Non-current regulatory assets	67,527	60,383	55,358
Pension and other postretirement benefit liabilities
Regulatory Assets [Line Items]
Non-current regulatory assets	$ 171,186	$ 179,824	$ 176,136