Significant Accounting Policies - Regulatory Liability Disclosure (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Regulatory Liabilities [Line Items]
Current regulatory liabilities	$ 34,275	$ 34,013	$ 28,041
Non-current regulatory liabilities	602,294	586,093	359,205
Gas costs
Regulatory Liabilities [Line Items]
Current regulatory liabilities	20,906	14,886	15,708
Non-current regulatory liabilities	3,460	4,630	2,719
Deferred Derivative Gain (Loss)
Regulatory Liabilities [Line Items]
Current regulatory liabilities	1,938	1,674	1,459
Non-current regulatory liabilities	1,077	1,306	162
Decoupling
Regulatory Liabilities [Line Items]
Current regulatory liabilities	2,153	322	134
Non-current regulatory liabilities	410	957	0
Income taxes
Regulatory Liabilities [Line Items]
Non-current regulatory liabilities	222,734	213,306	0
Accrued asset removal costs
Regulatory Liabilities [Line Items]
Non-current regulatory liabilities	370,245	360,929	350,828
Other
Regulatory Liabilities [Line Items]
Current regulatory liabilities	9,278	17,131	10,740
Non-current regulatory liabilities	$ 4,368	$ 4,965	$ 5,496