Short-Term Loans (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Short-Term Loans
Irrevocable letter of credit outstanding amount at the end of period					$ 23,500,000
Revolving credit facility outstanding amount at the end of period					53,650,000
Commitment fee paid to maintain credit facility (as a percent)	0.075%	0.02%
Maximum amount of outstanding short-term loans at any time during the period			104,000,000	139,700,000
Average daily balance of short-term loans outstanding during the period			$ 7,300,000	$ 17,700,000
Weighted average annual interest rates of average daily balance of short-term loans (as a percent)			1.81%	0.86%