Employee Benefits Pension and postretirement benefit plan disclosures (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Change in plan assets [Roll Forward]
Fair value of plan assets - ending balance	$ 423.9	$ 398.1
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Disclosures regarding the benefits provided under the plan	Pension plan benefits are based on the employee's age at retirement, years of service and highest earnings in a consecutive 72 semimonthly pay period during the 10 years preceding retirement.
Accumulated benefit obligation	519.6
Change in benefit obligation [Roll Forward]
Benefit obligation - beginning balance	492.4	486.6
Service cost	9.3	9.3	9.9
Interest cost	28.5	28.7	29.6
Change in plan assumptions	122.3	70.5
Actuarial loss	13.2	(11.0)
Curtailment charge	0	(18.3)
Transfer to QEP due to spinoff	0	(53.6)
Benefits paid	(18.8)	(19.8)
Benefit obligation - ending balance	646.9	492.4	486.6
Change in plan assets [Roll Forward]
Fair value of plan assets - beginning balance	359.9	318.6
Actual gain on plan assets	2.5	43.7
Company contributions to the plan	46.4	42.6
Transfer to QEP due to spinoff	0	(25.2)
Fair value of plan assets - ending balance	390.0	359.9	318.6
Funded status of plan [Abstract]
Underfunded status (current and long-term)	(256.9)	(132.5)
Estimated benefit-plan payments for the next five years and subsequent five years aggregated [Abstract]
2012	19.4
2013	20.5
2014	22.1
2015	26.5
2016	26.4
2017 through 2021	167.1
Components of pension and postretirement benefits expense [Abstract]
Service cost	9.3	9.3	9.9
Interest cost	28.5	28.7	29.6
Expected return on plan assets	(26.0)	(24.4)	(25.3)
Prior service and other costs	1.1	1.1	1.2
Recognized net actuarial loss	12.1	8.2	6.6
Cutailment charges	0.8	2.4	0
Special-termination benefits	0	0	2.0
Accretion of regulatory liability	0	0	0
Periodic expense	25.8	25.3	24.0
Pension Plans, Defined Benefit, Qualified Only [Member]
Assumptions used to calculate benefit obligations [Abstract]
Discount rate used to calculate benefit obligation (in hundredths)	4.80%	5.75%
Rate of increase in compensation used to calculate benefit obligation (in hundredths)	5.50%	4.00%
Long-term return on assets used to calculate benefit obligation (in hundredths)	7.25%	7.25%
Funded status of plan [Abstract]
Projected pension funding in the next fiscal year	53.6
Pension Plans, Defined Benefit, Nonqualified Only [Member]
Assumptions used to calculate benefit obligations [Abstract]
Discount rate used to calculate benefit obligation (in hundredths)	3.30%	5.75%
Rate of increase in compensation used to calculate benefit obligation (in hundredths)	5.50%	4.00%
Long-term return on assets used to calculate benefit obligation (in hundredths)	7.25%	7.25%
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Disclosures regarding the benefits provided under the plan	Provided to employees hired before January 1, 1997. The company pays a portion of the costs of health care benefits determined by an employee's years of service and generally limited to 170% of the 1992 contribution for employees who retired after January 1, 1993.
Period of time the transition obligation is being amortized over that began in 1992 (in years)	20
Assumptions used to calculate benefit obligations [Abstract]
Discount rate used to calculate benefit obligation (in hundredths)	4.40%	5.75%
Long-term return on assets used to calculate benefit obligation (in hundredths)	7.25%	7.25%
Assumed health care cost trend rates used to calculate benefit obligation [Abstract]
Health-care inflation rate used to calculate benefit obligation (in hundredths)	8.00%	8.00%
Ultimate health care cost trend rate used to calculate benefit obligation (in hundredths)	5.00%	5.00%
Year that rate reaches ultimate trend rate, used to calculate benefit obligation	2014	2013
Change in benefit obligation [Roll Forward]
Benefit obligation - beginning balance	77.9	76.3
Service cost	0.5	0.6	0.7
Interest cost	4.2	4.4	4.6
Change in plan assumptions	14.7	4.5
Actuarial loss	(0.9)	1.1
Curtailment charge	0	(4.4)
Transfer to QEP due to spinoff	0	0
Benefits paid	(7.7)	(4.6)
Benefit obligation - ending balance	88.7	77.9	76.3
Change in plan assets [Roll Forward]
Fair value of plan assets - beginning balance	38.4	35.8
Actual gain on plan assets	(0.7)	4.1
Company contributions to the plan	4.0	3.1
Transfer to QEP due to spinoff	0	0
Fair value of plan assets - ending balance	34.0	38.4	35.8
Funded status of plan [Abstract]
Underfunded status (current and long-term)	(54.7)	(39.5)
Estimated benefit-plan payments for the next five years and subsequent five years aggregated [Abstract]
2012	4.6
2013	4.7
2014	4.8
2015	4.9
2016	5.0
2017 through 2021	25.9
Components of pension and postretirement benefits expense [Abstract]
Service cost	0.5	0.6	0.7
Interest cost	4.2	4.4	4.6
Expected return on plan assets	(2.6)	(2.4)	(2.2)
Prior service and other costs	1.9	1.9	1.9
Recognized net actuarial loss	0.3	0.6	0.9
Cutailment charges	0	0.3	0
Special-termination benefits	0	0	0
Accretion of regulatory liability	0.8	0.8	0.8
Periodic expense	$ 5.1	$ 6.2	$ 6.7