Commitments and Contingent Liabilities (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Credit Facility 850 Million [Member]
Line of Credit Facility [Line Items]
Credit Facility Maturity Date	December 2015
Credit Facility Basis Of Fees	Senior unsecured long-term debt rating
Credit Facility Amount Utilized	$ 402,900,000	$ 183,500,000
Credit Facility 200 Million [Member]
Line of Credit Facility [Line Items]
Credit Facility Maturity Date	September 2019
Credit Facility Basis Of Fees	Fixed
Credit Facility Amount Utilized	200,000,000	200,000,000
Credit Facility 120 Million [Member]
Line of Credit Facility [Line Items]
Credit Facility Maturity Date	May 2016
Credit Facility Basis Of Fees	Fixed
Credit Facility Amount Utilized	$ 100,000,000	$ 0