Short-Term Loans (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2011
Short-Term Loans
Commitment fee paid to maintain credit facility (as a percent)	0.06%	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			$ 11.1
Weighted average annual interest rates of average daily balance of short-term loans (as a percent)			1.81%