Commitments and Contingent Liabilities (Credit Facility) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2014		Dec. 31, 2013
Credit Facility One Hundred Million [Member]
Line Of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 100.0
Maturity Date	June 2017
Amount Utilized	85	[1]	89	[1]
Basis Of Fees	Fixed
Credit Facility Fifty Three Million [Member]
Line Of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	79.0	[2]
Maturity Date	November 2014
Amount Utilized	79	[1]	58	[1]
Basis Of Fees	Fixed
Credit Facility 270 Million [Member]
Line Of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	270.0
Maturity Date	November 2017
Amount Utilized	270	[1]	270	[1]
Basis Of Fees	Fixed
Credit Facility 120 Million [Member]
Line Of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	120.0
Maturity Date	May 2016
Amount Utilized	80	[1]	85	[1]
Basis Of Fees	Fixed
Credit Facility 200 Million [Member]
Line Of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	200.0
Maturity Date	September 2019
Amount Utilized	200	[1]	200	[1]
Basis Of Fees	Fixed
Credit Facility 850 Million [Member]
Line Of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	850.0
Maturity Date	September 2019
Amount Utilized	178	[1]	68	[3]
Basis Of Fees	Senior unsecured long-term debt rating
Credit Facility One Hundred Fifty Million [Member]
Line Of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	97.0	[2]
Maturity Date	March 2019
Amount Utilized	97	[1]	133	[1]
Basis Of Fees	Fixed
Credit Facility One Hundred Fifty Million [Member]
Line Of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	150.0
Maturity Date	June 2016
Amount Utilized	150	[1]	0	[1]
Basis Of Fees	Fixed
Credit Facility Forty Seven Million [Member]
Line Of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	44.0	[2]
Maturity Date	May 2016
Amount Utilized	$ 0	[1]	$ 0	[1]
Basis Of Fees	Fixed

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

[2] Foreign currency facility, U.S. dollar amount may vary.

[3] Represents amount under expired syndicated credit facility.