Fair Value Disclosures (Summary Of Changes In Fair Value Of Financial Assets And Liabilities Classified As Level 3) (Details) - USD ($) $ in Thousands	10 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total gains (losses) (realized and unrealized)	$ (3,400)	$ 455,500	$ (24,800)
Total gains (losses) (realized and unrealized)	(5,800)	7,600	(14,900)
Secured financings issued	8,700
Corporate Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	38	38	38
Total gains (losses) (realized and unrealized)	0	0	0
Purchases	0	0	0
Sales	0	0	0
Settlements	0	0	0
Issuances		0	0
Net transfers into/ (out of) Level 3	0	0	0
Ending Balance	38	38	38
Change in unrealized gains/(losses) relating to instruments still held	0	0	0
Corporate Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		223	0
Total gains (losses) (realized and unrealized)		(110)	(149)
Purchases		(6,804)	(565)
Sales		6,691	960
Settlements		0	0
Issuances		0	0
Net transfers into/ (out of) Level 3		0	(23)
Ending Balance	0	0	223
Change in unrealized gains/(losses) relating to instruments still held		0	(8)
Residential Mortgage Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	1,542		0
Total gains (losses) (realized and unrealized)	(1,542)
Purchases	0
Sales	0
Settlements	0
Net transfers into/ (out of) Level 3	0
Ending Balance	0
Change in unrealized gains/(losses) relating to instruments still held	0
Net Derivative [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	11,185	(4,638)	6,905
Total gains (losses) (realized and unrealized)	4,408	(7,310)	15,055
Purchases	0	(6,705)	(24,682)
Sales	(300)	13,522	1,094
Settlements	(8,515)	37	322
Issuances		2,437	0
Net transfers into/ (out of) Level 3	127	2,415	(3,332)
Ending Balance	6,905	(242)	(4,638)
Change in unrealized gains/(losses) relating to instruments still held	1,609	4,754	(15,615)
Loans [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	7,398	14,450	22,462
Total gains (losses) (realized and unrealized)	2,959	(163)	0
Purchases	(16,027)	(2,059)	(18,332)
Sales	28,065	229	11,338
Settlements	67	0	0
Issuances		0	0
Net transfers into/ (out of) Level 3	0	(1,988)	(1,018)
Ending Balance	22,462	10,469	14,450
Change in unrealized gains/(losses) relating to instruments still held	(2,970)	104	0
Other Secured Financings [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		30,825	8,711
Total gains (losses) (realized and unrealized)		0	0
Purchases		0	0
Sales		0	0
Settlements		(15,704)	(17,525)
Issuances		36,995	39,639
Net transfers into/ (out of) Level 3		(51,572)	0
Ending Balance	8,711	544	30,825
Change in unrealized gains/(losses) relating to instruments still held		0	0
Corporate Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	13,234	20,964	9,884
Total gains (losses) (realized and unrealized)	1,551	11,154	957
Purchases	3,583	21,385	18,138
Sales	(7,141)	(6,391)	(12,826)
Settlements	0	0	0
Net transfers into (out of) Level 3	(1,343)	(6,206)	4,811
Ending Balance	9,884	40,906	20,964
Change in unrealized gains/(losses) relating to instruments still held	(419)	11,424	2,324
Corporate Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	31,820	22,766	25,666
Total gains (losses) (realized and unrealized)	(2,454)	(11,013)	6,629
Purchases	31,014	21,534	38,316
Sales	(34,125)	(14,636)	(40,328)
Settlements	0	0	0
Net transfers into (out of) Level 3	(589)	7,225	(7,517)
Ending Balance	25,666	25,876	22,766
Change in unrealized gains/(losses) relating to instruments still held	(2,749)	(9,443)	8,982
Collateralized Debt Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	24,736	124,650	37,216
Total gains (losses) (realized and unrealized)	(2,309)	(66,332)	(6,386)
Purchases	45,437	104,998	204,337
Sales	(32,874)	(107,381)	(181,757)
Settlements	0	(5,754)	(1,297)
Net transfers into (out of) Level 3	2,226	34,911	72,537
Ending Balance	37,216	85,092	124,650
Change in unrealized gains/(losses) relating to instruments still held	(8,384)	(48,514)	(1,141)
Municipal securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		0
Total gains (losses) (realized and unrealized)		10
Purchases		0
Sales		0
Settlements		(21,551)
Net transfers into (out of) Level 3		21,541
Ending Balance		0	0
Change in unrealized gains/(losses) relating to instruments still held		0
Sovereign securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		0
Total gains (losses) (realized and unrealized)		47
Purchases		1,032
Sales		(1,031)
Settlements		0
Net transfers into (out of) Level 3		72
Ending Balance		120	0
Change in unrealized gains/(losses) relating to instruments still held		39
U.S. Government And Federal Agency Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		0	0
Total gains (losses) (realized and unrealized)			13
Purchases			2,505
Sales			(2,518)
Settlements			0
Net transfers into (out of) Level 3			0
Ending Balance	0		0
Change in unrealized gains/(losses) relating to instruments still held			0
Residential Mortgage Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	169,426	82,557	105,492
Total gains (losses) (realized and unrealized)	(4,897)	(12,951)	(9,870)
Purchases	89,792	18,961	42,632
Sales	(150,807)	(31,762)	(61,689)
Settlements	(11,007)	(597)	(1,847)
Net transfers into (out of) Level 3	12,985	14,055	7,839
Ending Balance	105,492	70,263	82,557
Change in unrealized gains/(losses) relating to instruments still held	(6,932)	(4,498)	(4,679)
Commercial Mortgage Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	17,794	26,655	17,568
Total gains (losses) (realized and unrealized)	(4,469)	(3,813)	(4,237)
Purchases	20,130	3,480	49,159
Sales	(13,538)	(10,146)	(51,360)
Settlements	(100)	(6,861)	(782)
Net transfers into (out of) Level 3	(2,249)	5,011	16,307
Ending Balance	17,568	14,326	26,655
Change in unrealized gains/(losses) relating to instruments still held	(3,794)	(3,205)	(2,384)
Other Asset Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	1,292	2,294	12,611
Total gains (losses) (realized and unrealized)	(4,535)	(990)	1,784
Purchases	105,291	42,922	4,987
Sales	(104,711)	(1,299)	(18,002)
Settlements	0	(2)	0
Net transfers into (out of) Level 3	15,274	0	914
Ending Balance	12,611	42,925	2,294
Change in unrealized gains/(losses) relating to instruments still held	(3,497)	(254)	1,484
Loans And Other Receivables [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	170,986	97,258	145,890
Total gains (losses) (realized and unrealized)	15,008	(14,755)	(31,311)
Purchases	287,757	792,345	130,169
Sales	(115,231)	(576,536)	(92,140)
Settlements	(211,805)	(124,365)	(60,390)
Net transfers into (out of) Level 3	(825)	15,342	5,040
Ending Balance	145,890	189,289	97,258
Change in unrealized gains/(losses) relating to instruments still held	13,402	(16,802)	(26,864)
Investments at Fair Value [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	39,693	77,047	66,931
Total gains (losses) (realized and unrealized)	(1,317)	62,804	17,642
Purchases	28,515	5,510	32,493
Sales	(102)	(425)	(23,324)
Settlements	(875)	(4,093)	(1,243)
Net transfers into (out of) Level 3	1,017	58,951	(15,452)
Ending Balance	66,931	199,794	77,047
Change in unrealized gains/(losses) relating to instruments still held	$ (1,290)	(1,964)	1,985
Investment in FXCM [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		0
Total gains (losses) (realized and unrealized)		491,341
Purchases		279,000
Sales		0
Settlements		(144,652)
Net transfers into (out of) Level 3		0
Ending Balance		625,689	$ 0
Change in unrealized gains/(losses) relating to instruments still held		$ 491,341