Short-Term Borrowings (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Feb. 08, 2011
Short-term Debt [Line Items]
Maximum borrowing capacity		$ 250
Amount outstanding as of December 31, 2011	166.9
Short term borrowings maximum days outstanding	270 days
Weighted average interest rate as of December 31, 2011	0.57%
Daily average amount outstanding during 2011	83.4
Weighted average interest rate during 2011	0.42%
Maximum month-end balance during 2011	$ 166.9