Commitments and Contingent Liabilities (Credit Facility) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2013		Dec. 31, 2012
Credit Facility One Hundred Fifty Million [Member]
Line Of Credit Facility [Line Items]
Facility Capacity	$ 151.4
Maturity Date	March 2019
Amount Utilized	151.4	[1]	0	[1]
Basis Of Fees	Fixed
Credit Facility One Hundred Million [Member]
Line Of Credit Facility [Line Items]
Facility Capacity	100.0
Maturity Date	June 2017
Amount Utilized	92.2	[1]	0	[1]
Basis Of Fees	Fixed
Credit Facility Fifty Three Million [Member]
Line Of Credit Facility [Line Items]
Facility Capacity	52.9
Maturity Date	November 2013
Amount Utilized	52.9	[1]	0	[1]
Basis Of Fees	Fixed
Credit Facility 270 Million [Member]
Line Of Credit Facility [Line Items]
Facility Capacity	270.0
Maturity Date	November 2017
Amount Utilized	270.0	[1]	0	[1]
Basis Of Fees	Fixed
Credit Facility 120 Million [Member]
Line Of Credit Facility [Line Items]
Facility Capacity	120.0
Maturity Date	May 2016
Amount Utilized	115.0	[1]	100.0	[1]
Basis Of Fees	Fixed
Credit Facility 200 Million [Member]
Line Of Credit Facility [Line Items]
Facility Capacity	200.0
Maturity Date	September 2019
Amount Utilized	200.0	[1]	200.0	[1]
Basis Of Fees	Fixed
Credit Facility 850 Million [Member]
Line Of Credit Facility [Line Items]
Facility Capacity	850.0
Maturity Date	December 2015
Amount Utilized	$ 73.9	[1]	$ 402.9	[1]
Basis Of Fees	Senior unsecured long-term debt rating

[1] Represents issued but undrawn letters of credit. There was no cash borrowed for the periods presented.