Commitments and Contingent Liabilities (Narrative) (Details) $ in Millions	Sep. 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	$ 146.2	$ 176.5
Undrawn outstanding letters of credit between subsidiaries	$ 1,104.6	$ 1,035.0
Number of credit facilities | credit_facilities	7
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	6
Line of Credit Facility, Maximum Borrowing Capacity	$ 651.3