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Debt and Credit Facilities - Lines of Credit (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Line of Credit Facility [Line Items]    
Used lines of credit $ 23.2 $ 83.0
Unused lines of credit 1,108.6 1,074.4
Total available lines of credit 1,131.8 1,157.4
Cash held on deposit as compensating balance 0.0 $ 25.4
Committed Line Of Credit Facilities    
Line of Credit Facility [Line Items]    
Total available lines of credit $ 855.5