Benefit plans (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Benefit plans [Abstract]
Defined Contribution Plan, Employer Matching Contribution, Percent	50.00%
Defined Contribution Plan, Maximum Annual Contribution Per Employee, Percent	4.00%
Defined Benefit Plan, Contributions by Employer	$ 1.8	$ 1.8	$ 2.1
Health Benefit Insurance Expense	30.4	28.3	32.0
Liability for Unpaid Claims and Claims Adjustment Expense, Reported and Incurred but Not Reported (IBNR) Claims, Health Benefit Plan	3.1	3.4
Deferred Compensation Plan Employer Match, Salary	5.00%
Deferred Compensation Plan Employer Match, Bonus	5.00%
Deferred Compensation Arrangement with Individual, Employer Contribution	0.1	1.3	1.2
Deferred Compensation Plan Assets	0	15.3
Deferred Compensation Liability, Current	0	12.8
Deferred Compensation Arrangement with Individual, Compensation Expense	$ 1.2	$ 2.1	$ 1.4