Commitments and Contingencies (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011
Commitments for demand charges under existing long-term contracts and binding precedent agreements with various pipelines
Remedial action included in other credits	$ 1.7
Pipeline demand charges
Commitments for demand charges under existing long-term contracts and binding precedent agreements with various pipelines
Amount due as of the balance sheet date	2,331.4
Amount due in 2012	192.2
Amount due in 2013	165.2
Amount due in 2014	148.3
Amount due in 2015	178.7
Amount due in 2016	173.7
Amount due after 2017	1,473.3
Demand charges recoverable in customer rates, 2012	17.6
Demand charges recoverable in customer rates, 2013	3.5
Demand charges recoverable in customer rates, 2014	3.5
Demand charges recoverable in customer rates, 2015	3.5
Demand charges recoverable in customer rates, 2016	0.9
Capacity charges scheduled to be released, 2012	61.8
Capacity charges scheduled to be released, 2013	47.5
Capacity charges scheduled to be released, 2014	20.4
Drilling equipment and services supply agreements
Commitments for demand charges under existing long-term contracts and binding precedent agreements with various pipelines
Amount due as of the balance sheet date	$ 90.2