Short-Term Borrowings (Details) (USD $)	6 Months Ended	12 Months Ended
	Jun. 30, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Sep. 30, 2011
Daily borrowings under credit arrangements		$ 401,200,000
Compensating balance requirements	NO
Short-term borrowings from banks		199,400,000	109,600,000
Company-owned securities collateralized value			162,600,000
Company-owned securities collateralized value		392,395,000			392,400,000
Average bank borrowings		199,600,000	108,800,000	107,400,000
Weighted average daily interest rates		1.17%	1.05%
Debt Instrument, Interest Rate During Period			0.26%
Stock loan balance		124,711,000	27,907,000
Weighted average daily effective interest rate		1.28%	1.54%	1.07%
Secured Debt		680,000,000
Stifel Nicolaus [Member]
Debt Instrument, Interest Rate During Period		0.17%
Stock loan balance		124,700,000	27,900,000
Company Owned Securities [Member]
Company-owned securities collateralized value		$ 293,000,000