Collateralized Transactions (Contractual Maturity) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending arrangements	$ 2,670,611	$ 3,014,300
Repurchase agreements	14,978,982	20,158,422
Total	17,649,593	23,172,722
Overnight and Continuous [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending arrangements	1,391,681	1,522,475
Repurchase agreements	7,216,429	7,848,231
Total	8,608,110	9,370,706
Up to 30 Days [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending arrangements	0	0
Repurchase agreements	2,937,670	5,218,059
Total	2,937,670	5,218,059
30 to 90 Days [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending arrangements	1,278,930	973,201
Repurchase agreements	3,856,051	5,291,729
Total	5,134,981	6,264,930
Greater than 90 Days [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending arrangements	0	518,624
Repurchase agreements	968,832	1,800,403
Total	$ 968,832	$ 2,319,027