BUSINESS COMBINATIONS - Additional Information (Details) - USD ($)		6 Months Ended	12 Months Ended
	Mar. 11, 2016	Jun. 30, 2016	Dec. 31, 2015	Jun. 03, 2016	Jul. 17, 2015	Oct. 14, 2014
Business Acquisition [Line Items]
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Deposit Premium Percentage				3.00%
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Loan Held For Sale				$ 314,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accounts Payable				63,000,000
Business Acquisition Intangible Assets Expected Tax Deductible Amount				$ 13,000,000
Business Acquisition Intangible Assets Tax Deductible Period		15 years
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Other, Amount	$ 31,000,000
Business Loan Premium				0.50%
The Bank shares Inc [Member]
Business Acquisition [Line Items]
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Noncurrent Liabilities Deposits					$ 188,400,000
Business Combination Recognized Identifiable Assets Acquired and Liabilities Assumed Noncurrent Assets Loans					$ 111,300,000
Business Combination, Bargain Purchase, Gain Recognized, Amount			$ 416,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill, Total			$ 2,600,000
BMO Harris Central [Member]
Business Acquisition [Line Items]
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Noncurrent Liabilities Deposits						$ 0
Business Combination Recognized Identifiable Assets Acquired and Liabilities Assumed Noncurrent Assets Loans						$ 0