BORROWINGS - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Short-term advances	$ 1,700.0	$ 1,500.0
Short-term advances, percentage bearing fixed interest rate	80.50%	64.50%
Long-term borrowings, face amount	$ 558.0
Proceeds from debt, net of issuance costs	560.0
FHLB [Member]
Debt Instrument [Line Items]
Credit available with FHLB	8,100.0
Credit with FHLB utilized	$ 3,000.0
Minimum [Member] | FHLB [Member]
Debt Instrument [Line Items]
Effective interest rates	1.09%	1.62%
Maximum [Member] | FHLB [Member]
Debt Instrument [Line Items]
Effective interest rates	2.71%	2.71%
2.20% Senior Notes due February 24, 2023 [Member]
Debt Instrument [Line Items]
Long-term borrowings, face amount	$ 300.0
Proceeds from debt, net of issuance costs	$ 297.9
London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	240.00%