Short-Term Borrowings (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Short-term Debt [Line Items]
Commercial Paper, Balance	$ 122,934,000	$ 166,934,000
Maximum borrowing capacity under commercial paper program	250,000,000
Short term borrowings maximum days outstanding	270 days
Short-term Debt [Member]
Short-term Debt [Line Items]
Commercial Paper, Balance	122,900,000	166,900,000
Interest Rate, Commercial Paper	0.53%	0.57%
Maximum short-term debt outstanding	166,900,000	166,900,000
Average short-term debt outstanding	$ 78,900,000	$ 83,400,000
Weighted average interest rate	0.48%	0.42%