Debt (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Short-term Debt [Line Items]
Short-term debt outstanding	$ 382,640,000	$ 323,163,000
Commercial Paper [Member]
Short-term Debt [Line Items]
Short-term debt outstanding	382,600,000	323,200,000
Weighted average interest rates on short-term debt outstanding (in hundredths)	0.40%	0.50%
Average daily borrowings	$ 455,700,000	$ 404,900,000
Weighted average interest rates on average daily borrowings (in hundredths)	0.40%	0.40%