Fair Value Disclosures (Summary Of Changes In Fair Value Of Financial Assets And Liabilities Classified As Level 3) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total gains (losses) (realized and unrealized)	$ (48,900)	$ (5,300)	$ 508,900	$ 22,300
Total gains (losses) (realized and unrealized)	1,100	1,700	(2,900)	(10,100)
Corporate Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	38	38	38	38
Total gains (losses) (realized and unrealized)	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Issuances	0	0	0	0
Net transfers into/ (out of) Level 3	(38)	0	(38)	0
Ending Balance	0	38	0	38
Change in unrealized gains/(losses) relating to instruments still held	0	0	0	0
Corporate Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	452	2,780	223	0
Total gains (losses) (realized and unrealized)	(226)	(101)	(1)	163
Purchases	0	(2,566)	(6,677)	(7)
Sales	0	0	6,804	97
Settlements	0	0	0	0
Issuances	0	0	0	0
Net transfers into/ (out of) Level 3	0	129	(123)	(11)
Ending Balance	226	242	226	242
Change in unrealized gains/(losses) relating to instruments still held	226	67	(226)	163
Net Derivatives [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(1,586)	15,282	(4,638)	6,905
Total gains (losses) (realized and unrealized)	(1,020)	(1,632)	3,022	9,959
Purchases	(1,432)	(74)	(4,527)	(124)
Sales	11,618	74	11,340	(76)
Settlements	24	(70)	(30)	248
Issuances	416	0	1,901	0
Net transfers into/ (out of) Level 3	(857)	0	95	(3,332)
Ending Balance	7,163	13,580	7,163	13,580
Change in unrealized gains/(losses) relating to instruments still held	551	70	(5,211)	(10,519)
Loans [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	10,732	31,534	14,450	22,462
Total gains (losses) (realized and unrealized)	109	0	(102)	0
Purchases	(3,012)	(16,307)	(3,487)	(15,472)
Sales	0	0	0	3,549
Settlements	0	0	0	0
Issuances	0	0	0	0
Net transfers into/ (out of) Level 3	2,542	(8,566)	(490)	(3,878)
Ending Balance	10,371	6,661	10,371	6,661
Change in unrealized gains/(losses) relating to instruments still held	(110)	0	102	0
Other Secured Financings [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	56,060	20,288	30,825	8,711
Total gains (losses) (realized and unrealized)	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	(3,914)	(7,570)	(15,674)	(16,684)
Issuances	0	18,948	36,995	39,639
Net transfers into/ (out of) Level 3	(51,572)	0	(51,572)	0
Ending Balance	574	31,666	574	31,666
Change in unrealized gains/(losses) relating to instruments still held	0	0	0	0
Corporate Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	20,547	16,402	20,964	9,884
Total gains (losses) (realized and unrealized)	3,901	(480)	10,247	(1,528)
Purchases	21,162	4,528	22,631	36,661
Sales	(5,173)	(529)	(5,176)	(31,444)
Settlements	0	0	0	0
Net transfers into (out of) Level 3	(1,935)	(12,144)	(10,164)	(5,796)
Ending Balance	38,502	7,777	38,502	7,777
Change in unrealized gains/(losses) relating to instruments still held	3,803	(286)	10,210	(400)
Corporate Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	31,917	31,648	22,766	25,666
Total gains (losses) (realized and unrealized)	(5,276)	5,454	(5,425)	10,727
Purchases	10,395	21,793	83,613	137,164
Sales	(17,197)	(15,713)	(88,711)	(128,733)
Settlements	(1)	(34)	(1)	0
Net transfers into (out of) Level 3	4,493	(6,264)	12,089	(7,940)
Ending Balance	24,331	36,884	24,331	36,884
Change in unrealized gains/(losses) relating to instruments still held	(5,544)	3,470	(5,797)	10,177
Collateralized Debt Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	89,007	42,313	124,650	37,216
Total gains (losses) (realized and unrealized)	(12,560)	(845)	(28,999)	5,198
Purchases	14,961	7,613	63,038	94,743
Sales	0	(15,204)	(47,570)	(99,661)
Settlements	(13,230)	0	(20,481)	0
Net transfers into (out of) Level 3	2,872	9,863	(9,588)	6,244
Ending Balance	81,050	43,740	81,050	43,740
Change in unrealized gains/(losses) relating to instruments still held	(12,561)	(1,575)	(22,654)	(6,283)
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)		(11)		(11)
Purchases		2,505		2,505
Sales		0		0
Settlements		0		0
Net transfers into (out of) Level 3		0		0
Ending Balance		2,494		2,494
Change in unrealized gains/(losses) relating to instruments still held		(11)		(11)
Residential Mortgage-Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	88,695	71,962	82,557	105,492
Total gains (losses) (realized and unrealized)	(3,009)	(2,557)	(6,776)	(6,974)
Purchases	10,034	3,981	30,865	44,454
Sales	(8,424)	(9,635)	(25,222)	(65,229)
Settlements	(195)	(325)	(358)	(812)
Net transfers into (out of) Level 3	(679)	17,608	5,356	4,103
Ending Balance	86,422	81,034	86,422	81,034
Change in unrealized gains/(losses) relating to instruments still held	655	(302)	(2,507)	(3,564)
Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	17,862	24,246	26,655	17,568
Total gains (losses) (realized and unrealized)	(510)	(256)	(2,053)	(3,120)
Purchases	0	641	3,366	34,959
Sales	(680)	(7,068)	(9,973)	(32,774)
Settlements	0	0	(6,981)	(1,315)
Net transfers into (out of) Level 3	(1,525)	1,764	4,133	4,009
Ending Balance	15,147	19,327	15,147	19,327
Change in unrealized gains/(losses) relating to instruments still held	(545)	(832)	(1,851)	(3,380)
Other Asset-Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	11,857	45,444	2,294	12,611
Total gains (losses) (realized and unrealized)	870	1,272	666	256
Purchases	21,913	50,620	69,892	52,495
Sales	0	(49,411)	(40,000)	(52,282)
Settlements	(1,167)	(8,774)	(1,438)	(8,804)
Net transfers into (out of) Level 3	(877)	(37,072)	1,182	(2,197)
Ending Balance	32,596	2,079	32,596	2,079
Change in unrealized gains/(losses) relating to instruments still held	813	(3)	607	0
Loans and other receivables [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	108,756	138,643	97,258	145,890
Total gains (losses) (realized and unrealized)	(2,111)	(8,074)	(7,331)	(9,028)
Purchases	31,269	194,387	115,370	247,383
Sales	(603)	(96,340)	(40,978)	(147,851)
Settlements	(42,529)	(40,617)	(82,100)	(61,791)
Net transfers into (out of) Level 3	617	26	13,180	13,422
Ending Balance	95,399	188,025	95,399	188,025
Change in unrealized gains/(losses) relating to instruments still held	(6,182)	(7,967)	(8,850)	(8,961)
Investments at Fair Value [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	154,862	101,697	77,047	66,931
Total gains (losses) (realized and unrealized)	82,943	198	87,254	26,744
Purchases	0	500	0	28,160
Sales	(3,000)	(5,414)	(427)	(18,100)
Settlements	(277)	(305)	(3,818)	(945)
Net transfers into (out of) Level 3	(21,010)	5,416	53,462	(698)
Ending Balance	213,518	102,092	213,518	102,092
Change in unrealized gains/(losses) relating to instruments still held	27,623	$ 307	32,016	$ 12,255
Investment in FXCM [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	759,000		0
Total gains (losses) (realized and unrealized)	(113,193)		461,341
Purchases	0		279,000
Sales	0		0
Settlements	(32,807)		(127,341)
Net transfers into (out of) Level 3	0		0
Ending Balance	613,000		613,000
Change in unrealized gains/(losses) relating to instruments still held	$ (113,193)		$ 461,341