Related Parties (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Related parties
Total rental expense under multiple leases	$ 18.5	$ 15.8	$ 13.8
Certain shareholders
Related parties
Total rental expense under multiple leases	2.4	2.4	2.1
Law firm
Related parties
Expenses incurred	3.2	2.9	1.1
Number of directors who are also partner (in persons)	1
Financial services firm
Related parties
Expenses incurred	$ 0.5	$ 0.3	$ 0.6
Number of directors who are also partner (in persons)	1