Fair Value Disclosures - Schedule of Assets and Liabilities Measured on Recurring Basis at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value based on NAV	$ 20,200	$ 24,300
Assets:
Counterparty/cash-collateral netting, Assets	(3,318,481)	(4,255,998)
Total trading assets, excluding investments at fair value based on NAV	15,492,572	14,960,928
Total available for sale securities	716,561	301,049
Liabilities:
Counterparty/cash-collateral netting, Liabilities	(3,490,514)	(4,229,213)
Total trading liabilities	8,454,965	8,388,619
Short-term borrowings	23,324
Other secured financings		41,768
Long-term debt	606,956	248,856
Trading liabilities
Liabilities:
Corporate equity securities	1,753,437	1,610,667
Corporate debt securities	1,689,347	1,718,947
U.S. government and federal agency securities	1,430,737	976,497
Sovereign obligations	2,173,635	2,629,344
Loans	1,152,310	802,355
Derivatives	255,394	650,809
Counterparty/cash-collateral netting, Liabilities	(3,490,514)	(4,229,213)
Trading assets
Assets:
Corporate equity securities	3,058,033	2,637,555
Corporate debt securities	3,287,336	2,700,025
Collateralized debt obligations and collateralized loan obligations	181,350	108,660
U.S. government and federal agency securities	1,308,673	2,446,123
Municipal securities	710,513	735,726
Sovereign obligations	2,417,459	2,423,048
Loans and other receivables	1,667,885	1,639,105
Derivatives	179,488	371,078
Counterparty/cash-collateral netting, Assets	(3,318,481)	(4,255,998)
Investments at fair value	330,890	314,359
FXCM term loan	72,800	164,500
Available for sale securities
Assets:
Corporate equity securities	88,486	79,425
Corporate debt securities		179
U.S. government and federal agency securities	552,805	174,933
Residential mortgage-backed securities
Assets:
Total available for sale securities	34,561	19,133
Residential mortgage-backed securities | Trading assets
Assets:
Mortgage- and asset-backed securities, assets	1,479,371	999,266
Residential mortgage-backed securities | Available for sale securities
Assets:
Mortgage- and asset-backed securities, assets	34,561	19,133
Commercial mortgage-backed securities
Assets:
Total available for sale securities	5,870	8,337
Commercial mortgage-backed securities | Trading liabilities
Liabilities:
Commercial mortgage-backed securities	105
Commercial mortgage-backed securities | Trading assets
Assets:
Mortgage- and asset-backed securities, assets	520,534	316,985
Commercial mortgage-backed securities | Available for sale securities
Assets:
Mortgage- and asset-backed securities, assets	5,870	8,337
Other asset-backed securities
Assets:
Total available for sale securities	34,839	19,042
Other asset-backed securities | Trading assets
Assets:
Mortgage- and asset-backed securities, assets	278,240	104,498
Other asset-backed securities | Available for sale securities
Assets:
Mortgage- and asset-backed securities, assets	34,839	19,042
Level 1
Assets:
Total trading assets, excluding investments at fair value based on NAV	5,791,641	6,348,755
Total available for sale securities	641,291	254,358
Liabilities:
Total trading liabilities	4,618,008	3,948,201
Short-term borrowings	0
Other secured financings		0
Long-term debt	0	0
Level 1 | Trading liabilities
Liabilities:
Corporate equity securities	1,721,267	1,593,548
Corporate debt securities	0	0
U.S. government and federal agency securities	1,430,737	976,497
Sovereign obligations	1,216,643	1,375,590
Loans	0	0
Derivatives	249,361	2,566
Level 1 | Trading assets
Assets:
Corporate equity securities	2,975,463	2,522,977
Corporate debt securities	0	0
Collateralized debt obligations and collateralized loan obligations	0	0
U.S. government and federal agency securities	1,269,230	2,389,397
Municipal securities	0	0
Sovereign obligations	1,381,552	1,432,556
Loans and other receivables	0	0
Derivatives	165,396	3,825
Investments at fair value	0	0
FXCM term loan	0	0
Level 1 | Available for sale securities
Assets:
Corporate equity securities	88,486	79,425
Corporate debt securities		0
U.S. government and federal agency securities	552,805	174,933
Level 1 | Residential mortgage-backed securities | Trading assets
Assets:
Mortgage- and asset-backed securities, assets	0	0
Level 1 | Residential mortgage-backed securities | Available for sale securities
Assets:
Mortgage- and asset-backed securities, assets	0	0
Level 1 | Commercial mortgage-backed securities | Trading liabilities
Liabilities:
Commercial mortgage-backed securities	0
Level 1 | Commercial mortgage-backed securities | Trading assets
Assets:
Mortgage- and asset-backed securities, assets	0	0
Level 1 | Commercial mortgage-backed securities | Available for sale securities
Assets:
Mortgage- and asset-backed securities, assets	0	0
Level 1 | Other asset-backed securities | Trading assets
Assets:
Mortgage- and asset-backed securities, assets	0	0
Level 1 | Other asset-backed securities | Available for sale securities
Assets:
Mortgage- and asset-backed securities, assets	0	0
Level 2
Assets:
Total trading assets, excluding investments at fair value based on NAV	12,369,954	12,072,393
Total available for sale securities	75,270	46,691
Liabilities:
Total trading liabilities	7,307,269	8,658,547
Short-term borrowings	23,324
Other secured financings		41,350
Long-term debt	606,956	248,856
Level 2 | Trading liabilities
Liabilities:
Corporate equity securities	32,122	16,806
Corporate debt securities	1,688,825	1,718,424
U.S. government and federal agency securities	0	0
Sovereign obligations	956,992	1,253,754
Loans	1,148,824	801,977
Derivatives	3,480,506	4,867,586
Level 2 | Trading assets
Assets:
Corporate equity securities	60,300	92,839
Corporate debt securities	3,261,300	2,675,020
Collateralized debt obligations and collateralized loan obligations	139,166	54,306
U.S. government and federal agency securities	39,443	56,726
Municipal securities	710,513	708,469
Sovereign obligations	1,035,907	990,492
Loans and other receivables	1,620,581	1,557,233
Derivatives	3,323,278	4,616,822
Investments at fair value	946	0
FXCM term loan	0	0
Level 2 | Available for sale securities
Assets:
Corporate equity securities	0	0
Corporate debt securities		179
U.S. government and federal agency securities	0	0
Level 2 | Residential mortgage-backed securities | Trading assets
Assets:
Mortgage- and asset-backed securities, assets	1,453,294	960,494
Level 2 | Residential mortgage-backed securities | Available for sale securities
Assets:
Mortgage- and asset-backed securities, assets	34,561	19,133
Level 2 | Commercial mortgage-backed securities | Trading liabilities
Liabilities:
Commercial mortgage-backed securities	0
Level 2 | Commercial mortgage-backed securities | Trading assets
Assets:
Mortgage- and asset-backed securities, assets	508,115	296,405
Level 2 | Commercial mortgage-backed securities | Available for sale securities
Assets:
Mortgage- and asset-backed securities, assets	5,870	8,337
Level 2 | Other asset-backed securities | Trading assets
Assets:
Mortgage- and asset-backed securities, assets	217,111	63,587
Level 2 | Other asset-backed securities | Available for sale securities
Assets:
Mortgage- and asset-backed securities, assets	34,839	19,042
Level 3
Assets:
Total trading assets, excluding investments at fair value based on NAV	649,458	795,778
Total available for sale securities	0	0
Liabilities:
Total trading liabilities	20,202	11,084
Short-term borrowings	0
Other secured financings		418
Long-term debt	0	0
Level 3 | Trading liabilities
Liabilities:
Corporate equity securities	48	313
Corporate debt securities	522	523
U.S. government and federal agency securities	0	0
Sovereign obligations	0	0
Loans	3,486	378
Derivatives	16,041	9,870
Level 3 | Trading assets
Assets:
Corporate equity securities	22,270	21,739
Corporate debt securities	26,036	25,005
Collateralized debt obligations and collateralized loan obligations	42,184	54,354
U.S. government and federal agency securities	0	0
Municipal securities	0	27,257
Sovereign obligations	0	0
Loans and other receivables	47,304	81,872
Derivatives	9,295	6,429
Investments at fair value	329,944	314,359
FXCM term loan	72,800	164,500
Level 3 | Available for sale securities
Assets:
Corporate equity securities	0	0
Corporate debt securities		0
U.S. government and federal agency securities	0	0
Level 3 | Residential mortgage-backed securities | Trading assets
Assets:
Mortgage- and asset-backed securities, assets	26,077	38,772
Level 3 | Residential mortgage-backed securities | Available for sale securities
Assets:
Mortgage- and asset-backed securities, assets	0	0
Level 3 | Commercial mortgage-backed securities | Trading liabilities
Liabilities:
Commercial mortgage-backed securities	105
Level 3 | Commercial mortgage-backed securities | Trading assets
Assets:
Mortgage- and asset-backed securities, assets	12,419	20,580
Level 3 | Commercial mortgage-backed securities | Available for sale securities
Assets:
Mortgage- and asset-backed securities, assets	0	0
Level 3 | Other asset-backed securities | Trading assets
Assets:
Mortgage- and asset-backed securities, assets	61,129	40,911
Level 3 | Other asset-backed securities | Available for sale securities
Assets:
Mortgage- and asset-backed securities, assets	$ 0	$ 0