FAIR VALUE, Recurring and Nonrecurring Fair Value Measurements (Details) (USD $)	3 Months Ended	6 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2015	Mar. 31, 2014	Sep. 30, 2014
Assets, Fair Value Disclosure
Total debt securities	$ 696,813,000	$ 696,813,000		$ 602,973,000
Derivative contracts	41,717,000	41,717,000		28,205,000
Equity securities	42,383,000	42,383,000		34,142,000
Corporate loans	0	0		990,000
Other	16,618,000	16,618,000		13,083,000
Total trading instruments	797,531,000	797,531,000		679,393,000
Available for sale securities	531,940,000	531,940,000		562,289,000
Derivative instruments associated with offsetting matched book positions	421,850,000	421,850,000		323,337,000
Other real estate owned (OREO)	6,500,000	6,500,000		5,400,000
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	310,179,000	310,179,000		215,142,000
Derivative liability	26,022,000	26,022,000		12,372,000
Equity securities	34,388,000	34,388,000		10,886,000
Other securities	1,751,000	1,751,000		0
Trading instruments sold but not yet purchased	372,340,000	372,340,000		238,400,000
Derivative instruments associated with offsetting matched book positions	421,850,000	421,850,000		323,337,000
Transfers of Financial Instruments into (out of) Level 1 and 2 [Abstract]
Fair value of financial instruments, level 1 to level 2 transfers	600,000	1,100,000		800,000
Fair value of financial instruments, level 2 to level 1 transfers		1,100,000		1,300,000
Adjustments to fair value of nonrecurring fair value measurements [Abstract]
Additional provision for loan losses due to fair value adjustment		222,000	176,000
Other losses due to fair value adjustment		149,000	1,500,000
Recurring
Assets, Fair Value Disclosure
Total debt securities	696,813,000	696,813,000		602,973,000
Netting adjustments	(79,697,000)	(79,697,000)		(61,718,000)
Equity securities	42,383,000	42,383,000		34,142,000
Corporate loans				990,000
Other	16,618,000	16,618,000		13,083,000
Total trading instruments	797,531,000	797,531,000		679,393,000
Available for sale securities	531,940,000	531,940,000		562,289,000
Private equity investments	220,944,000	220,944,000		211,666,000
Other investments	208,976,000	208,976,000		215,751,000
Derivative instruments associated with offsetting matched book positions	421,850,000	421,850,000		323,337,000
Deposits with clearing organizations	23,592,000	23,592,000
Other assets	2,196,000	2,196,000		787,000
Total other assets	10,196,000	10,196,000		3,249,000
Total assets at fair value on a recurring basis	2,215,029,000	2,215,029,000		1,995,685,000
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	310,179,000	310,179,000		215,142,000
Netting adjustments	(79,028,000)	(79,028,000)		(63,296,000)
Equity securities	34,388,000	34,388,000		10,886,000
Other securities	1,751,000	1,751,000
Trading instruments sold but not yet purchased	372,340,000	372,340,000		238,400,000
Derivative instruments associated with offsetting matched book positions	421,850,000	421,850,000		323,337,000
Other liabilities	58,000	58,000		58,000
Trade and other payables	2,539,000	2,539,000
Total liabilities at fair value on a recurring basis	796,729,000	796,729,000		561,795,000
Recurring | Municipal and provincial obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	4,876,000	4,876,000		11,647,000
Recurring | Corporate obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	18,648,000	18,648,000		15,333,000
Recurring | Government obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	284,821,000	284,821,000		187,424,000
Recurring | Agency MBS and CMOs
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	1,834,000	1,834,000		738,000
Recurring | MK&Co
Adjustments to fair value of nonrecurring fair value measurements [Abstract]
Other investments with obligations to perform under deferred compensation plan	143,000,000	143,000,000		144,000,000
Recurring | Interest rate contract
Assets, Fair Value Disclosure
Netting adjustments	(79,697,000)	(79,697,000)		(61,718,000)
Derivative contracts	41,717,000	41,717,000		28,205,000
Liabilities, Fair Value Disclosure [Abstract]
Netting adjustments	(79,028,000)	(79,028,000)		(63,296,000)
Derivative liability	26,022,000	26,022,000		12,372,000
Recurring | Forward foreign exchange contracts
Assets, Fair Value Disclosure
Netting adjustments	0	0		0
Derivative contracts	8,000,000	8,000,000		2,462,000
Liabilities, Fair Value Disclosure [Abstract]
Netting adjustments	0	0
Derivative liability	2,481,000	2,481,000
Recurring | Municipal and provincial obligations
Assets, Fair Value Disclosure
Total debt securities	244,519,000	244,519,000		203,889,000
Recurring | Corporate obligations
Assets, Fair Value Disclosure
Total debt securities	96,073,000	96,073,000		111,928,000
Recurring | Government and agency obligations
Assets, Fair Value Disclosure
Total debt securities	164,008,000	164,008,000		101,362,000
Recurring | Agency MBS and CMOs
Assets, Fair Value Disclosure
Total debt securities	140,403,000	140,403,000		127,419,000
Available for sale securities	240,488,000	240,488,000		267,720,000
Recurring | Non-agency CMOs
Assets, Fair Value Disclosure
Available for sale securities	87,362,000	87,362,000		91,918,000
Recurring | Other securities
Assets, Fair Value Disclosure
Available for sale securities	2,028,000	2,028,000		1,916,000
Recurring | Non-agency CMOs and ABS
Assets, Fair Value Disclosure
Total debt securities	51,810,000	51,810,000		58,375,000
Recurring | ARS - municipals
Assets, Fair Value Disclosure
Available for sale securities	89,614,000	89,614,000		86,696,000
Recurring | ARS - preferred securities
Assets, Fair Value Disclosure
Available for sale securities	112,448,000	112,448,000		114,039,000
Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets, Fair Value Disclosure
Total debt securities	60,174,000	60,174,000		21,019,000
Equity securities	36,790,000	36,790,000		28,834,000
Corporate loans				0
Other	657,000	657,000		566,000
Total trading instruments	97,621,000	97,621,000		50,419,000
Available for sale securities	2,028,000	2,028,000		1,916,000
Private equity investments	0	0		0
Other investments	206,888,000	206,888,000		212,753,000
Derivative instruments associated with offsetting matched book positions	0	0		0
Deposits with clearing organizations	23,592,000	23,592,000
Other assets	0	0		0
Total other assets	0	0		0
Total assets at fair value on a recurring basis	330,129,000	330,129,000		265,088,000
Total bank loans, net	0	0		0
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	291,170,000	291,170,000		199,284,000
Equity securities	34,387,000	34,387,000		10,884,000
Other securities	2,000	2,000
Trading instruments sold but not yet purchased	325,559,000	325,559,000		210,168,000
Derivative instruments associated with offsetting matched book positions	0	0		0
Other liabilities	0	0		0
Trade and other payables	0	0
Total liabilities at fair value on a recurring basis	325,559,000	325,559,000		210,168,000
Recurring | Quoted prices in active markets for identical assets (Level 1) | Municipal and provincial obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	4,482,000	4,482,000		11,093,000
Recurring | Quoted prices in active markets for identical assets (Level 1) | Corporate obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	33,000	33,000		29,000
Recurring | Quoted prices in active markets for identical assets (Level 1) | Government obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	284,821,000	284,821,000		187,424,000
Recurring | Quoted prices in active markets for identical assets (Level 1) | Agency MBS and CMOs
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	1,834,000	1,834,000		738,000
Recurring | Quoted prices in active markets for identical assets (Level 1) | Interest rate contract
Assets, Fair Value Disclosure
Derivative contracts	0	0		0
Liabilities, Fair Value Disclosure [Abstract]
Derivative contracts	0	0		0
Recurring | Quoted prices in active markets for identical assets (Level 1) | Forward foreign exchange contracts
Assets, Fair Value Disclosure
Derivative contracts	0	0		0
Liabilities, Fair Value Disclosure [Abstract]
Derivative contracts	0	0
Recurring | Quoted prices in active markets for identical assets (Level 1) | Municipal and provincial obligations
Assets, Fair Value Disclosure
Total debt securities	4,955,000	4,955,000		11,407,000
Recurring | Quoted prices in active markets for identical assets (Level 1) | Corporate obligations
Assets, Fair Value Disclosure
Total debt securities	15,899,000	15,899,000		1,989,000
Recurring | Quoted prices in active markets for identical assets (Level 1) | Government and agency obligations
Assets, Fair Value Disclosure
Total debt securities	38,717,000	38,717,000		7,376,000
Recurring | Quoted prices in active markets for identical assets (Level 1) | Agency MBS and CMOs
Assets, Fair Value Disclosure
Total debt securities	603,000	603,000		247,000
Available for sale securities	0	0		0
Recurring | Quoted prices in active markets for identical assets (Level 1) | Non-agency CMOs
Assets, Fair Value Disclosure
Available for sale securities	0	0		0
Recurring | Quoted prices in active markets for identical assets (Level 1) | Other securities
Assets, Fair Value Disclosure
Available for sale securities	2,028,000	2,028,000		1,916,000
Recurring | Quoted prices in active markets for identical assets (Level 1) | Non-agency CMOs and ABS
Assets, Fair Value Disclosure
Total debt securities	0	0		0
Recurring | Quoted prices in active markets for identical assets (Level 1) | ARS - municipals
Assets, Fair Value Disclosure
Available for sale securities	0	0		0
Recurring | Quoted prices in active markets for identical assets (Level 1) | ARS - preferred securities
Assets, Fair Value Disclosure
Available for sale securities	0	0		0
Recurring | Significant other observable inputs (Level 2)
Assets, Fair Value Disclosure
Total debt securities	636,629,000	636,629,000		581,943,000
Equity securities	5,579,000	5,579,000		5,264,000
Corporate loans				990,000
Other	15,181,000	15,181,000		10,208,000
Total trading instruments	778,803,000	778,803,000		688,328,000
Available for sale securities	327,850,000	327,850,000		359,638,000
Private equity investments	0	0		0
Other investments	1,172,000	1,172,000		1,267,000
Derivative instruments associated with offsetting matched book positions	421,850,000	421,850,000		323,337,000
Deposits with clearing organizations	0	0
Other assets	0	0		0
Total other assets	8,000,000	8,000,000		2,462,000
Total assets at fair value on a recurring basis	1,537,675,000	1,537,675,000		1,375,032,000
Total bank loans, net	38,848,000	38,848,000		23,678,000
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	19,009,000	19,009,000		15,858,000
Equity securities	1,000	1,000		2,000
Other securities	1,749,000	1,749,000
Trading instruments sold but not yet purchased	125,809,000	125,809,000		91,528,000
Derivative instruments associated with offsetting matched book positions	421,850,000	421,850,000		323,337,000
Other liabilities	0	0		0
Trade and other payables	2,481,000	2,481,000
Total liabilities at fair value on a recurring basis	550,140,000	550,140,000		414,865,000
Recurring | Significant other observable inputs (Level 2) | Municipal and provincial obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	394,000	394,000		554,000
Recurring | Significant other observable inputs (Level 2) | Corporate obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	18,615,000	18,615,000		15,304,000
Recurring | Significant other observable inputs (Level 2) | Government obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	0	0		0
Recurring | Significant other observable inputs (Level 2) | Agency MBS and CMOs
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	0	0		0
Recurring | Significant other observable inputs (Level 2) | Interest rate contract
Assets, Fair Value Disclosure
Derivative contracts	121,414,000	121,414,000		89,923,000
Liabilities, Fair Value Disclosure [Abstract]
Derivative contracts	105,050,000	105,050,000		75,668,000
Recurring | Significant other observable inputs (Level 2) | Forward foreign exchange contracts
Assets, Fair Value Disclosure
Derivative contracts	8,000,000	8,000,000		2,462,000
Liabilities, Fair Value Disclosure [Abstract]
Derivative contracts	2,481,000	2,481,000
Recurring | Significant other observable inputs (Level 2) | Municipal and provincial obligations
Assets, Fair Value Disclosure
Total debt securities	239,564,000	239,564,000		192,482,000
Recurring | Significant other observable inputs (Level 2) | Corporate obligations
Assets, Fair Value Disclosure
Total debt securities	80,174,000	80,174,000		109,939,000
Recurring | Significant other observable inputs (Level 2) | Government and agency obligations
Assets, Fair Value Disclosure
Total debt securities	125,291,000	125,291,000		93,986,000
Recurring | Significant other observable inputs (Level 2) | Agency MBS and CMOs
Assets, Fair Value Disclosure
Total debt securities	139,800,000	139,800,000		127,172,000
Available for sale securities	240,488,000	240,488,000		267,720,000
Recurring | Significant other observable inputs (Level 2) | Non-agency CMOs
Assets, Fair Value Disclosure
Available for sale securities	87,362,000	87,362,000		91,918,000
Recurring | Significant other observable inputs (Level 2) | Other securities
Assets, Fair Value Disclosure
Available for sale securities	0	0		0
Recurring | Significant other observable inputs (Level 2) | Non-agency CMOs and ABS
Assets, Fair Value Disclosure
Total debt securities	51,800,000	51,800,000		58,364,000
Recurring | Significant other observable inputs (Level 2) | ARS - municipals
Assets, Fair Value Disclosure
Available for sale securities	0	0		0
Recurring | Significant other observable inputs (Level 2) | ARS - preferred securities
Assets, Fair Value Disclosure
Available for sale securities	0	0		0
Recurring | Significant unobservable inputs (level 3)
Assets, Fair Value Disclosure
Total debt securities	10,000	10,000		11,000
Equity securities	14,000	14,000		44,000
Corporate loans				0
Other	780,000	780,000		2,309,000
Total trading instruments	804,000	804,000		2,364,000
Available for sale securities	202,062,000	202,062,000		200,735,000
Private equity investments	220,944,000	220,944,000		211,666,000
Other investments	916,000	916,000		1,731,000
Derivative instruments associated with offsetting matched book positions	0	0		0
Deposits with clearing organizations	0	0
Other assets	2,196,000	2,196,000		787,000
Total other assets	2,196,000	2,196,000		787,000
Total assets at fair value on a recurring basis	426,922,000	426,922,000		417,283,000
Total bank loans, net	11,854,431,000	11,854,431,000		10,738,136,000
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	0	0		0
Equity securities	0	0		0
Other securities	0	0
Trading instruments sold but not yet purchased	0	0		0
Derivative instruments associated with offsetting matched book positions	0	0		0
Other liabilities	58,000	58,000		58,000
Trade and other payables	58,000	58,000
Total liabilities at fair value on a recurring basis	58,000	58,000		58,000
Auction rate securities [Abstract]
Jefferson County, Alabama Limited Obligation School Warrants ARS	62,000,000	62,000,000		58,000,000
Private Equity Investments [Abstract]
Portion of significant private equity investments included in noncontrolling interests	56,000,000	56,000,000		55,000,000
Portion of private equity investments owned by parent	165,000,000	165,000,000		157,000,000
Weighted-average ownership percentage (in hundredths)	75.00%	75.00%		74.00%
Recurring | Significant unobservable inputs (level 3) | Municipal and provincial obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	0	0		0
Recurring | Significant unobservable inputs (level 3) | Corporate obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	0	0		0
Recurring | Significant unobservable inputs (level 3) | Government obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	0	0		0
Recurring | Significant unobservable inputs (level 3) | Agency MBS and CMOs
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	0	0		0
Recurring | Significant unobservable inputs (level 3) | Interest rate contract
Assets, Fair Value Disclosure
Derivative contracts	0	0		0
Liabilities, Fair Value Disclosure [Abstract]
Derivative contracts	0	0		0
Recurring | Significant unobservable inputs (level 3) | Forward foreign exchange contracts
Assets, Fair Value Disclosure
Derivative contracts	0	0		0
Liabilities, Fair Value Disclosure [Abstract]
Derivative contracts	0	0
Recurring | Significant unobservable inputs (level 3) | Municipal and provincial obligations
Assets, Fair Value Disclosure
Total debt securities	0	0		0
Recurring | Significant unobservable inputs (level 3) | Corporate obligations
Assets, Fair Value Disclosure
Total debt securities	0	0		0
Recurring | Significant unobservable inputs (level 3) | Government and agency obligations
Assets, Fair Value Disclosure
Total debt securities	0	0		0
Recurring | Significant unobservable inputs (level 3) | Agency MBS and CMOs
Assets, Fair Value Disclosure
Total debt securities	0	0		0
Available for sale securities	0	0		0
Recurring | Significant unobservable inputs (level 3) | Non-agency CMOs
Assets, Fair Value Disclosure
Available for sale securities	0	0		0
Recurring | Significant unobservable inputs (level 3) | Other securities
Assets, Fair Value Disclosure
Available for sale securities	0	0		0
Recurring | Significant unobservable inputs (level 3) | Non-agency CMOs and ABS
Assets, Fair Value Disclosure
Total debt securities	10,000	10,000		11,000
Recurring | Significant unobservable inputs (level 3) | ARS - municipals
Assets, Fair Value Disclosure
Available for sale securities	89,614,000	89,614,000		86,696,000
Recurring | Significant unobservable inputs (level 3) | ARS - preferred securities
Assets, Fair Value Disclosure
Available for sale securities	112,448,000	112,448,000		114,039,000
Nonrecurring
Assets, Fair Value Disclosure
Total bank loans, net	131,140,000	131,140,000		112,938,000
Other real estate owned (OREO)	1,196,000	1,196,000		768,000
Total assets at fair value on a nonrecurring basis	132,336,000	132,336,000		113,706,000
Nonrecurring | Impaired loans
Assets, Fair Value Disclosure
Total bank loans, net	82,010,000	82,010,000		90,327,000
Nonrecurring | Loans held for sale
Assets, Fair Value Disclosure
Total bank loans, net	49,130,000	49,130,000		22,611,000
Nonrecurring | Quoted prices in active markets for identical assets (Level 1)
Assets, Fair Value Disclosure
Total bank loans, net	0	0		0
Other real estate owned (OREO)	0	0		0
Total assets at fair value on a nonrecurring basis	0	0		0
Nonrecurring | Quoted prices in active markets for identical assets (Level 1) | Impaired loans
Assets, Fair Value Disclosure
Total bank loans, net	0	0		0
Nonrecurring | Quoted prices in active markets for identical assets (Level 1) | Loans held for sale
Assets, Fair Value Disclosure
Total bank loans, net	0	0		0
Nonrecurring | Significant other observable inputs (Level 2)
Assets, Fair Value Disclosure
Total bank loans, net	79,696,000	79,696,000		57,410,000
Other real estate owned (OREO)	1,196,000	1,196,000		768,000
Total assets at fair value on a nonrecurring basis	80,892,000	80,892,000		58,178,000
Nonrecurring | Significant other observable inputs (Level 2) | Impaired loans
Assets, Fair Value Disclosure
Total bank loans, net	30,566,000	30,566,000		34,799,000
Nonrecurring | Significant other observable inputs (Level 2) | Loans held for sale
Assets, Fair Value Disclosure
Total bank loans, net	49,130,000	49,130,000		22,611,000
Nonrecurring | Significant unobservable inputs (level 3)
Assets, Fair Value Disclosure
Total bank loans, net	51,444,000	51,444,000		55,528,000
Other real estate owned (OREO)	0	0		0
Total assets at fair value on a nonrecurring basis	51,444,000	51,444,000		55,528,000
Nonrecurring | Significant unobservable inputs (level 3) | Impaired loans
Assets, Fair Value Disclosure
Total bank loans, net	51,444,000	51,444,000		55,528,000
Nonrecurring | Significant unobservable inputs (level 3) | Loans held for sale
Assets, Fair Value Disclosure
Total bank loans, net	$ 0	$ 0		$ 0