Benefit plans (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Benefit plans [Abstract]
Defined Contribution Plan Employer Match	50.00%
Defined Contribution Plan Employer Match Maximum	4.00%
Defined Contribution Plan, Contributions by Employer	$ 1.8	$ 2.1	$ 2.2
Health Benefit Insurance Expense	28.3	32.0	27.3
Liability for Unpaid Claims and Claims Adjustment Expense, Reported and Incurred but Not Reported (IBNR) Claims, Health Benefit Plan	3.4	4.1
Deferred Compensation Plan Employer Match, Salary	5.00%
Deferred Compensation Plan Employer Match, Bonus	5.00%
Deferred Compensation Arrangement with Individual, Employer Contribution	1.3	1.2	0.9
Deferred Compensation Plan Assets	15.3	18.2
Deferred Compensation Liability, Current	12.8
Deferred Compensation Liability, Classified, Noncurrent		15.7
Deferred Compensation Arrangement with Individual, Compensation Expense	$ 2.1	$ 1.4	$ 0.8