Significant Accounting Policies - Regulatory Liability Disclosure (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	$ 34,946	$ 34,013	$ 33,211
Non-current regulatory liabilities	600,442	586,093	357,587
Gas costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	17,798	14,886	13,741
Non-current regulatory liabilities	5,639	4,630	4,740
Deferred Derivative Gain (Loss) [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	1,120	1,674	2,870
Non-current regulatory liabilities	1,148	1,306	46
Decoupling
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	2,501	322	0
Non-current regulatory liabilities	1,253	957	0
Estimated Regulatory Liability [Member]
Regulatory Liabilities [Line Items]
Non-current regulatory liabilities	219,795	213,306	0
Accrued asset removal costs
Regulatory Liabilities [Line Items]
Non-current regulatory liabilities	365,363	360,929	345,614
Other
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	13,527	17,131	16,600
Non-current regulatory liabilities	$ 7,244	$ 4,965	$ 7,187