BORROWINGS - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Short-term advances	$ 1,400,000,000		$ 1,400,000,000	$ 1,500,000,000
Short-term advances, percentage bearing fixed interest rate	100.00%		100.00%	64.50%
Loss on extinguishment of debt and related hedge termination costs	$ 13,300,000
Long-term borrowings, face amount	558,000,000		$ 558,000,000
Proceeds from debt, net of issuance costs			560,000,000
FHLB [Member]
Debt Instrument [Line Items]
Credit available with FHLB	8,400,000,000		8,400,000,000
Credit with FHLB utilized	2,100,000,000		$ 2,100,000,000
Reduction of higher rate outstanding	$ 415,000,000.0
Minimum [Member] | FHLB [Member]
Debt Instrument [Line Items]
Effective interest rates	0.30%		0.30%	1.62%
Maximum [Member] | FHLB [Member]
Debt Instrument [Line Items]
Effective interest rates	2.45%		2.45%	2.71%
2.20% Senior Notes due February 24, 2023 [Member]
Debt Instrument [Line Items]
Long-term borrowings, face amount	$ 300,000,000	$ 300,000,000	$ 300,000,000
Proceeds from debt, net of issuance costs		$ 297,900,000	$ 298,000,000