LOANS AND ALLOWANCE FOR CREDIT LOSSES (Lending Asset Quality Indicators) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 35,683,403	[1]	$ 32,169,154	[1]
Commercial [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	34,023,949	[1]	30,426,897	[1]
Commercial [Member] | Special mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	677,485	[1]	640,834	[1]
Commercial [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	882,278	[1]	977,431	[1]
Commercial [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	99,691	[1]	123,992	[1]
Corporate banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,306,009		3,767,870
Corporate banking [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,985,287		3,385,253
Corporate banking [Member] | Special mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	98,326		106,001
Corporate banking [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	188,516		260,122
Corporate banking [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	33,880		16,494
Middle market commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,547,840		3,510,371
Middle market commercial real estate [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,221,349		3,068,666
Middle market commercial real estate [Member] | Special mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	67,147		62,062
Middle market commercial real estate [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	213,643		310,318
Middle market commercial real estate [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	45,701		69,325
Santander real estate capital [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,832,100		2,025,644
Santander real estate capital [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,611,551		1,779,544
Santander real estate capital [Member] | Special mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	145,138		112,257
Santander real estate capital [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	74,673		132,999
Santander real estate capital [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	738		844
Commercial and industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	16,082,765		12,838,349
Commercial and industrial [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	15,495,626		12,320,254
Commercial and industrial [Member] | Special mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	241,729		283,939
Commercial and industrial [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	326,900		202,347
Commercial and industrial [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	18,510		31,809
Multi-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	8,941,466		8,237,029
Multi-family [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	8,767,111		8,109,023
Multi-family [Member] | Special mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	121,038		59,379
Multi-family [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	53,007		63,284
Multi-family [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	310		5,343
Remaining commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,973,223		1,789,891
Remaining commercial [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,943,025		1,764,157
Remaining commercial [Member] | Special mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,107		17,196
Remaining commercial [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	25,539		8,361
Remaining commercial [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	552		177
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	39,120,473	[1]	17,881,472	[1]
Consumer [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	37,866,392	[1]	17,255,401	[1]
Consumer [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,254,081	[1]	626,071	[1]
Residential mortgages [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	7,525,709		9,672,204
Residential mortgages [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	7,280,915		9,198,639
Residential mortgages [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	244,794		473,565
Home equity loans and lines of credit [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,215,554		6,311,694
Home equity loans and lines of credit [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,072,029		6,203,419
Home equity loans and lines of credit [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	143,525		108,275
Retail installment contracts and auto loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	21,870,891		81,804
Retail installment contracts and auto loans [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	21,053,587		79,936
Retail installment contracts and auto loans [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	817,304		1,868
Personal unsecured loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,131,371		493,785
Personal unsecured loan [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,129,569		493,785
Personal unsecured loan [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,802		0
Other consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,376,948		1,321,985
Other consumer [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,330,292		1,279,622
Other consumer [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 46,656		$ 42,363

[1] Financing receivables include LHFS.