Allowance for Doubtful Accounts (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Summary of allowance for doubtful accounts
Balance at beginning of year	$ 0.5	$ 0.7	$ 0.7
Provision charged to expense	0.6	0.7	0.6
Write-offs, less recoveries	(0.5)	(0.9)	(0.6)
Balance at end of year	$ 0.6	$ 0.5	$ 0.7