Accounts receivable and allowance for doubtful accounts (Details) (USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2010
Accounts receivable and allowance for doubtful accounts [Abstract]
Total balance of receivables past due 90 days or more	$ 41.4	$ 21.6
Allowance for doubtful accounts	$ 14.2	$ 15.3	$ 14.9