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DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES - Derivative Assets and Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Feb. 07, 2017
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	$ 2,797,967	$ 2,510,937
Estimated fair value asset (liability)	4,520	3,470
Interest rate swaps
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	10,600
Total cash flow hedges | Interest rate swaps | FHLB borrowings
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	300,000		$ 300,000
Total economic hedges
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	2,500,000
Total economic hedges | Interest rate swaps
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	2,000,000
Total economic hedges | Risk participation agreements with dealer banks
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	200,000
Total economic hedges | Forward sale commitments
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	300,000
Designated as Hedging Instrument | Total cash flow hedges
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	0	0
Estimated fair value asset (liability)	0	0
Designated as Hedging Instrument | Total cash flow hedges | Interest rate swaps | FHLB borrowings
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	$ 0	$ 0
Weighted average maturity	0 years	0 years
Weighted average rate, received (as percent)	0.00%	0.00%
Weighted average rate, contract pay rate (as percent)	0.00%	0.00%
Estimated fair value asset (liability)	$ 0	$ 0
Designated as Hedging Instrument | Total economic hedges
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	2,501,720	2,316,971
Estimated fair value asset (liability)	(2,011)	(1,789)
Designated as Hedging Instrument | Total economic hedges | Interest rate swaps | Tax advantaged economic development bonds
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	$ 10,590	$ 10,755
Weighted average maturity	11 years 8 months 15 days	11 years 10 months 24 days
Weighted average rate, received (as percent)	2.03%	1.73%
Weighted average rate, contract pay rate (as percent)	5.09%	5.09%
Estimated fair value asset (liability)	$ (1,338)	$ (1,649)
Designated as Hedging Instrument | Total economic hedges | Interest rate swaps | Loans with commercial loan customers
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	$ 1,006,921	$ 943,795
Weighted average maturity	5 years 9 months 12 days	5 years 10 months 12 days
Weighted average rate, received (as percent)	3.54%	3.26%
Weighted average rate, contract pay rate (as percent)	4.33%	4.25%
Estimated fair value asset (liability)	$ 11,474	$ (3,195)
Designated as Hedging Instrument | Total economic hedges | Reverse interest rate swaps | Loans with commercial loan customers
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	$ 1,006,921	$ 943,795
Weighted average maturity	5 years 9 months 12 days	5 years 10 months 12 days
Weighted average rate, received (as percent)	4.33%	4.25%
Weighted average rate, contract pay rate (as percent)	3.54%	3.26%
Estimated fair value asset (liability)	$ (11,228)	$ 3,204
Designated as Hedging Instrument | Total economic hedges | Risk participation agreements with dealer banks
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	$ 167,795	$ 142,054
Weighted average maturity	7 years	8 years 4 months 20 days
Estimated fair value asset (liability)	$ (10)	$ (26)
Designated as Hedging Instrument | Total economic hedges | Forward sale commitments
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	$ 309,493	$ 276,572
Weighted average maturity	2 months 12 days	2 months 12 days
Estimated fair value asset (liability)	$ (909)	$ (123)
Not Designated as Hedging Instrument
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	296,247	193,966
Estimated fair value asset (liability)	6,531	5,259
Not Designated as Hedging Instrument | Commitments to lend
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	$ 296,247	$ 193,966
Weighted average maturity	2 months 12 days	2 months 12 days
Estimated fair value asset (liability)	$ 6,531	$ 5,259

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+ References

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- Definition

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+ References

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- Definition

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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- Definition

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

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