Summary of Significant Accounting Policies Regulatory Assets Disclosure (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012		Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	$ 384,453		$ 382,255		$ 362,472	$ 362,290	$ 364,132		$ 367,536	$ 325,692	$ 323,371	$ 343,085
Regulatory Assets, Current	39,001		52,448				90,490
Other Regulatory [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	9,376	[1]	7,239	[1]			13,720	[1]
Regulatory Assets, Current	18,304	[1]	24,405	[1]			21,302	[1]
Unrealized Loss On Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	642	[2]	578	[2]			3,947	[2]
Regulatory Assets, Current	3,450	[2]	10,796	[2]			53,697	[2]
Pension Balancing [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	17,322	[3]	14,727	[3]			8,367	[3]
Income Tax Asset [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	53,065		55,879				63,452
Pension and Other Postretirement Benefit Liabilities [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	178,377	[3]	182,688	[3]			166,639	[3]
Regulatory Assets, Current	17,247	[3]	17,247	[3]			15,491	[3]
Environmental Restoration Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	$ 125,671	[3]	$ 121,144	[3]			$ 108,007	[3]

[1] 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.

[2] 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.

[3] Certain utility pension costs are approved for regulatory deferral, including amounts recorded to the pension balancing account, to mitigate the effects of higher and lower pension expenses. Pension costs that are deferred include an interest component when recognized in net periodic benefit costs. See Note 7.