INCOME TAXES - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Income Tax Contingency [Line Items]
Minimum percentage of bank net taxable income for paying Income tax at normal rate	20.00%
Income tax (benefit) expense 1	$ (3.0)	$ 17.6
Effective tax rate maximum statutory rate	37.50%
Estimated annual effective tax rate, excluding entities from which a tax benefit cannot be recognized and discrete items	24.00%	28.00%
Estimated annual effective tax rate, including all entities	26.00%	29.00%
Effective Income Tax Rate Excluding Discrete Items	25.00%	26.00%
Increase in Net Deferred Tax Assets	$ 31.3
Deferred Tax Assets Net	307.8
Valuation allowance	88.5
COVID-19
Income Tax Contingency [Line Items]
Income tax (benefit) expense 1	(20.0)
United States [Member]
Income Tax Contingency [Line Items]
Income tax (benefit) expense 1	1.0	$ 1.8
FirstBank [Member]
Income Tax Contingency [Line Items]
Deferred Tax Assets Net	307.7		$ 264.8
Valuation allowance	$ 52.7		$ 55.6
Domestic Tax Authority [Member] | Other taxable domestic corporations [Member]
Income Tax Contingency [Line Items]
Dividend received deduction	85.00%
Domestic Tax Authority [Member] | Subsidiaries [Member]
Income Tax Contingency [Line Items]
Dividend received deduction	100.00%