Significant Accounting Policies - Regulatory Liability Disclosure (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	$ 34,659	$ 40,290	$ 27,921
Non-current regulatory liabilities	363,838	349,319	342,143
Gas costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	16,459	8,054	12,001
Non-current regulatory liabilities	1,015	1,021	765
Deferred Derivative Gain (Loss) [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	2,020	16,624	4,857
Non-current regulatory liabilities	1,555	3,265	1,151
Other
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	15,866	15,612	11,063
Non-current regulatory liabilities	5,162	3,926	3,528
Accrued asset removal costs
Regulatory Liabilities [Line Items]
Non-current regulatory liabilities	$ 356,106	$ 341,107	$ 336,699