Summary of Significant Accounting Policies Regulatory Liabilities Disclosure (Details) - USD ($) $ in Thousands		Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Regulatory Liabilities [Line Items]
Regulatory liabilities		$ 334,490	$ 317,205	$ 312,500
Current Regulatory Liabilities		34,127	19,105	23,352
Gas Costs Payable [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		6,357	2,507	410
Current Regulatory Liabilities		22,499	5,700	6,704
Unrealized Gain On Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	[1]	299	0	602
Current Regulatory Liabilities	[1]	2,939	240	5,320
Other Regulatory [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	[2]	3,367	3,460	3,673
Current Regulatory Liabilities	[2]	8,689	13,165	11,328
Accrued Asset Removal Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	[3]	$ 324,467	$ 311,238	$ 307,815

[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. See Note 2 of the 2014 Form 10-K