Significant Accounting Policies - Regulatory Asset Disclosure (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Regulatory Assets [Line Items]
Regulatory Liability, Current	$ 34,659	$ 40,290	$ 27,921
Current regulatory assets	49,504	42,362	55,280
Non-current regulatory assets	345,352	357,530	341,188
Annual tariff rider collection	5,000
Deferred Derivative Gain (Loss) [Member]
Regulatory Assets [Line Items]
Current regulatory assets	8,887	1,315	5,205
Non-current regulatory assets	3,926	913	1,682
Gas costs
Regulatory Assets [Line Items]
Current regulatory assets	1,851	6,830	10,164
Non-current regulatory assets	48	89	712
Environmental Restoration Costs
Regulatory Assets [Line Items]
Current regulatory assets	6,362	9,989	9,734
Non-current regulatory assets	63,339	63,970	64,279
Other
Regulatory Assets [Line Items]
Current regulatory assets	16,741	11,161	14,149
Non-current regulatory assets	10,137	14,130	10,484
Decoupling
Regulatory Assets [Line Items]
Current regulatory assets	15,663	13,067	16,028
Non-current regulatory assets	1,025	5,860	1,006
Pension balancing
Regulatory Assets [Line Items]
Non-current regulatory assets	57,599	50,863	48,637
Income taxes
Regulatory Assets [Line Items]
Non-current regulatory assets	36,591	38,670	40,106
Pension and other postretirement benefit liabilities
Regulatory Assets [Line Items]
Non-current regulatory assets	172,687	183,035	174,282
Decoupling
Regulatory Assets [Line Items]
Regulatory Liability, Current	$ 314	$ 0	$ 0