Summary of Significant Accounting Policies Regulatory Liability Disclosure (Details) - USD ($) $ in Thousands		Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Regulatory Liabilities [Line Items]
Current Regulatory Liabilities		$ 33,211	$ 40,290	$ 35,596
Regulatory liabilities		357,587	349,319	346,761
Gas Costs Payable [Member]
Regulatory Liabilities [Line Items]
Current Regulatory Liabilities		13,741	8,054	22,098
Regulatory liabilities		4,740	1,021	9,221
Unrealized Gain On Derivatives [Member]
Regulatory Liabilities [Line Items]
Current Regulatory Liabilities	[1]	2,870	16,624	1,960
Regulatory liabilities	[1]	46	3,265	452
Other Regulatory [Member]
Regulatory Liabilities [Line Items]
Current Regulatory Liabilities	[2]	16,600	15,612	11,538
Regulatory liabilities	[2]	7,187	3,926	6,088
Asset Removal Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	[3]	$ 345,614	$ 341,107	$ 331,000

[1] Unrealized gains or losses on derivatives are non-cash items and, therefore, do not earn a rate of return or a carrying charge. These amounts are recoverable through utility rates as part of the annual Purchased Gas Adjustment (PGA) mechanism when realized at settlement.

[2] These balances primarily consist of deferrals and amortizations under approved regulatory mechanisms. The accounts being amortized typically earn a rate of return or carrying charge.

[3] Estimated costs of removal on certain regulated properties are collected through rates.