Short-Term Borrowings (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($) item	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) item	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Short-term Debt [Line Items]
Uncommitted secured lines of credit			$ 780,000
Number of banks | item	4		4
Daily borrowings under credit arrangements	$ 543,300		$ 543,300
Compensating balances	0		0
Committed revolving credit facility	100,000		100,000
Outstanding on our revolving credit facility	0		0
Short-term borrowings from banks	$ 543,289		$ 543,289
Weighted average daily interest rates	1.00%		1.00%
Average outstanding securities	$ 31,100	$ 252,300	$ 48,500	$ 175,400
Debt instrument interest rate during period	1.02%	1.12%	1.00%	1.13%
Stifel Nicolaus [Member]
Short-term Debt [Line Items]
Short-term borrowings from banks	$ 543,300		$ 543,300		$ 0
Average outstanding securities	$ 43,000	$ 68,100	$ 39,100	$ 70,500
Debt instrument interest rate during period	0.29%	0.19%	0.27%	0.18%
Company Owned Securities [Member]
Short-term Debt [Line Items]
Company-owned securities collateralized value	$ 748,100		$ 748,100
Revolving Credit Facility [Member]
Short-term Debt [Line Items]
LIBOR rate			2.00%