Future minimum lease payments (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Rent Obligations, due 2013	$ 309.1
Rent Obligations, due 2014	275.8
Rent Obligations, due 2015	243.4
Rent Obligations, due 2016	200.3
Rent Obligations, due 2017	164.2
Rent Obligations, due thereafter	675.8
Total Rent Obligations	1,868.6
Sublease Rental Income, due 2013	(26.9)
Sublease Rental Income, due 2014	(14.7)
Sublease Rental Income, due 2015	(7.3)
Sublease Rental Income, due 2016	(2.1)
Sublease Rental Income, due 2017	(0.6)
Sublease Rental Income, due thereafter	(0.3)
Total Sublease Rental Income	(51.9)
Net Rent, due 2013	282.2
Net Rent, due 2014	261.1
Net Rent, due 2015	236.1
Net Rent, due 2016	198.2
Net Rent, due 2017	163.6
Net Rent, due thereafter	675.5
Total Net Rent	$ 1,816.7

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