Commitments and Contingent Liabilities (Narrative) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014 credit_facilities	Dec. 31, 2013
Commitments And Contingent Liabilities [Line Items]
Undrawn outstanding letters of credit in favor of third parties	$ 212.5	$ 210.3
Undrawn outstanding letters of credit between subsidiaries	1,137.7	995.5
Number of credit facilities	9
Syndicated revolving credit facility capacity	850.0
Guarantee Of Subsidiary Collateral Financing [Member]
Commitments And Contingent Liabilities [Line Items]
Guarantor Obligations, Current Carrying Value	300.0
Letter of Credit [Member]
Commitments And Contingent Liabilities [Line Items]
Number of credit facilities	8
Facility Capacity	$ 1,080.5