Natural gas costs recoverable or refundable through rate adjustments (Details 10) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Minimum period over which natural gas costs greater or less than amounts being recovered become recoverable or refundable through rate adjustments	12 months
Maximum period over which natural gas costs greater or less than amounts being recovered become recoverable or refundable through rate adjustments	28 months
Natural gas costs refundable	$ 35.3	$ 45.1
Natural gas costs recoverable	$ 3.0	$ 2.6