Commitments and Contingent Liabilities (Narrative) (Details) $ in Millions	Jun. 30, 2015 USD ($) credit_facilities	Dec. 31, 2014 USD ($)
Commitments And Contingent Liabilities [Line Items]
Undrawn outstanding letters of credit in favor of third parties	$ 147.2	$ 176.5
Undrawn outstanding letters of credit between subsidiaries	$ 1,159.5	$ 1,035.0
Number of credit facilities | credit_facilities	8
Credit Facility 850 Million [Member]
Commitments And Contingent Liabilities [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 850.0
Letter of Credit [Member]
Commitments And Contingent Liabilities [Line Items]
Number of credit facilities | credit_facilities	7
Line of Credit Facility, Maximum Borrowing Capacity	$ 806.7