SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Detail) - USD ($) $ in Millions		9 Months Ended
	Jan. 01, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity attributable to parent		$ 4,951.0	$ 4,897.0	$ 4,883.0	$ 4,820.0	$ 4,753.0	$ 4,608.0
Initial ACL on PCD Loans	$ 50.3	$ 50.3
Impact of ASC 326 Adoption [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity attributable to parent				(50.0)
Commercial Loan [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Threshold period past due for non-accrual loans		90 days
Installment Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Threshold period past due for non-accrual loans		120 days
Residential Mortgages and Consumer Lines of Credit [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Threshold period past due for non-accrual loans		180 days
Retained Earnings [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity attributable to parent		$ 838.0	$ 796.0	798.0	$ 744.0	$ 683.0	$ 576.0
Retained Earnings [Member] | Impact of ASC 326 Adoption [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity attributable to parent				$ (50.0)
Retained Earnings [Member] | Accounting Standards Update 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity attributable to parent	(51.0)
Retained Earnings [Member] | Accounting Standards Update 2016-13 [Member] | Impact of ASC 326 Adoption [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity attributable to parent	$ (50.6)
Minimum [Member] | Commercial Loan [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing receivable, individual reserves in default, threshold		1.0
Maximum [Member] | Commercial and Consumer Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing receivable, individual reserves in default, threshold		$ 1.0