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Short-Term Loans (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2012
Mar. 31, 2011
Short-Term Loans    
Commitment fee paid to maintain credit facility (as a percent) 0.0769% 0.05%
Maximum amount of outstanding short-term loans at any time during the period   $ 104.0
Average daily balance of short-term loans outstanding during the period   $ 22.3
Weighted average annual interest rates of average daily balance of short-term loans (as a percent)   1.81%