Collateralized Transactions (Collateral Pledged) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending arrangements	$ 2,930,376	$ 3,014,300
Repurchase agreements	16,013,959	20,158,422
Total	18,944,335	23,172,722
Corporate equity securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending arrangements	2,341,506	2,200,273
Repurchase agreements	174,077	271,519
Total	2,515,583	2,471,792
Corporate Debt Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending arrangements	578,451	779,044
Repurchase agreements	1,701,732	1,721,583
Total	2,280,183	2,500,627
Mortgage- and asset-backed securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending arrangements	0	0
Repurchase agreements	2,358,531	3,537,812
Total	2,358,531	3,537,812
U.S. government and federal agency securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending arrangements	10,419	34,983
Repurchase agreements	8,759,548	12,003,521
Total	8,769,967	12,038,504
Municipal securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending arrangements	0	0
Repurchase agreements	420,957	357,350
Total	420,957	357,350
Sovereign securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending arrangements	0	0
Repurchase agreements	2,120,421	1,804,103
Total	2,120,421	1,804,103
Loans and other receivables [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending arrangements	0	0
Repurchase agreements	478,693	462,534
Total	$ 478,693	$ 462,534