Significant Accounting Policies - Regulatory Liability Disclosure (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Regulatory Liabilities [Line Items]
Current regulatory liabilities	$ 37,504	$ 34,013	$ 34,659
Regulatory Liability, Noncurrent	606,175	586,093	363,838
Gas costs
Regulatory Liabilities [Line Items]
Current regulatory liabilities	20,716	14,886	16,459
Regulatory Liability, Noncurrent	1,409	4,630	1,015
Deferred Derivative Gain (Loss)
Regulatory Liabilities [Line Items]
Current regulatory liabilities	2,862	1,674	2,020
Regulatory Liability, Noncurrent	861	1,306	1,555
Decoupling
Regulatory Liabilities [Line Items]
Current regulatory liabilities	1,697	322	314
Regulatory Liability, Noncurrent	119	957	0
Income taxes
Regulatory Liabilities [Line Items]
Regulatory Liability, Noncurrent	223,841	213,306	0
Accrued asset removal costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Noncurrent	375,257	360,929	356,106
Other
Regulatory Liabilities [Line Items]
Current regulatory liabilities	12,229	17,131	15,866
Regulatory Liability, Noncurrent	$ 4,688	$ 4,965	$ 5,162