Summary of Significant Accounting Policies Regulatory Liability Disclosure (Details) - USD ($) $ in Thousands		Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Regulatory Liabilities [Line Items]
Current Regulatory Liabilities		$ 35,596	$ 29,927	$ 24,263
Regulatory liabilities		346,761	339,287	326,424
Gas Costs Payable [Member]
Regulatory Liabilities [Line Items]
Current Regulatory Liabilities		22,098	14,157	12,774
Regulatory liabilities		9,221	8,869	4,729
Unrealized Gain On Derivatives [Member]
Regulatory Liabilities [Line Items]
Current Regulatory Liabilities	[1]	1,960	2,659	436
Regulatory liabilities	[1]	452	27	117
Other Regulatory [Member]
Regulatory Liabilities [Line Items]
Current Regulatory Liabilities	[2]	11,538	13,111	11,053
Regulatory liabilities	[2]	6,088	3,344	5,632
Asset Removal Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	[3]	$ 331,000	$ 327,047	$ 315,946

[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.