Components of Regulatory Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 373,319	$ 364,181
Removal Costs Recovered through Rates
Regulatory Liabilities [Line Items]
Regulatory liabilities	301,537	285,901
Other Regulatory Liability
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 71,782	$ 78,280