Significant Accounting Policies - Regulatory Liability Disclosure (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Regulatory Liabilities [Line Items]
Current regulatory liabilities	$ 28,041	$ 40,290	$ 27,300
Non-current regulatory liabilities	359,205	349,319	341,259
Gas costs
Regulatory Liabilities [Line Items]
Current regulatory liabilities	15,708	8,054	12,501
Non-current regulatory liabilities	2,719	1,021	1,622
Unrealized loss on derivatives
Regulatory Liabilities [Line Items]
Current regulatory liabilities	1,459	16,624	7,428
Non-current regulatory liabilities	162	3,265	3,541
Other
Regulatory Liabilities [Line Items]
Current regulatory liabilities	10,874	15,612	7,371
Non-current regulatory liabilities	5,496	3,926	3,469
Accrued asset removal costs
Regulatory Liabilities [Line Items]
Non-current regulatory liabilities	$ 350,828	$ 341,107	$ 332,627