Summary of Significant Accounting Policies Regulatory Liabilities Disclosure (details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012		Jun. 30, 2012
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 294,202		$ 288,113		$ 280,295
Current Regulatory Liabilities	16,644		20,792		20,748
Gas Costs Payable [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	481		0		1,504
Current Regulatory Liabilities	6,353		9,100		12,980
Unrealized Gain On Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,054	[1]	3,639	[1]	1,170	[1]
Current Regulatory Liabilities	547	[1]	1,950	[1]	2,142	[1]
Other Regulatory [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	3,562	[2]	3,261	[2]	2,865	[2]
Current Regulatory Liabilities	9,744	[2]	9,742	[2]	5,626	[2]
Accrued Asset Removal Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 289,105		$ 281,213		$ 274,756

[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] Other primarily consists of several deferrals and amortizations under other approved regulatory mechanisms. The accounts being amortized typically earn a rate of return or carrying charge.