Summary of Significant Accounting Policies Regulatory Liability Disclosure (Details) - USD ($) $ in Thousands		Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015
Regulatory Liabilities [Line Items]
Current Regulatory Liabilities		$ 27,300	$ 29,927	$ 26,712
Regulatory liabilities		341,259	339,287	328,646
Gas Costs Payable [Member]
Regulatory Liabilities [Line Items]
Current Regulatory Liabilities		12,501	14,157	20,087
Regulatory liabilities		1,622	8,869	3,615
Unrealized Gain On Derivatives [Member]
Regulatory Liabilities [Line Items]
Current Regulatory Liabilities	[1]	7,428	2,659	1,015
Regulatory liabilities	[1]	3,541	27	1,369
Other Regulatory [Member]
Regulatory Liabilities [Line Items]
Current Regulatory Liabilities	[2]	7,371	13,111	5,610
Regulatory liabilities	[2]	3,469	3,344	3,456
Asset Removal Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	[3]	$ 332,627	$ 327,047	$ 320,206

[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.