Summary of Significant Accounting Policies Regulatory Liabilities Disclosure (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014		Mar. 31, 2014
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 326,424		$ 317,205		$ 308,858
Current Regulatory Liabilities	24,263		19,105		37,686
Gas Costs Payable [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	4,729		2,507		2,602
Current Regulatory Liabilities	12,774		5,700		9,137
Unrealized Gain On Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	117	[1]	0	[1]	1,078	[1]
Current Regulatory Liabilities	436	[1]	240	[1]	15,788	[1]
Other Regulatory [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	5,632	[2]	3,460	[2]	6,152	[2]
Current Regulatory Liabilities	11,053	[2]	13,165	[2]	12,761	[2]
Accrued Asset Removal Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 315,946	[3]	$ 311,238	[3]	$ 299,026	[3]

[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.