Debt (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Base rate
Debt
Reference rate for variable rate of interest	Base rate
Basis points added to reference rate (as a percent)	1.00%
Eurodollar rate
Debt
Reference rate for variable rate of interest	Eurodollar
Basis points added to reference rate (as a percent)	2.00%
Credit facility
Debt
Maximum borrowing capacity	$ 800.0
Maximum additional borrowings available at the entity's request subject satisfaction of certain conditions	150.0
Commitment fee on the unused portion of the credit facility (as a percent)	0.25%
Amount outstanding	100.5
Letters of credit and bankers' acceptances
Debt
Maximum borrowing capacity	300.0
Letters of credit fees (as a percent)	2.25%
Letters of credit, outstanding amount	47.4	45.3
Term Loans
Debt
Maximum borrowing capacity	247.5	250.0
Other uncommitted credit lines
Debt
Maximum borrowing capacity	219.5
Letters of credit, outstanding amount	$ 184.2	$ 122.3