Low Income Housing Tax Credits and Other Tax Credit Investments - Narrative (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Investments In Affordable Housing Projects [Line Items]
Tax credit investments	$ 64.1	$ 59.8
Renewable Energy Program
Investments In Affordable Housing Projects [Line Items]
Equity investments	1.2
Outstanding investment commitments	$ 1.7	$ 1.7
Low income housing tax credit and other tax credit investments
Investments In Affordable Housing Projects [Line Items]
Tax credit period of benefits	15 years
Operating loss tax benefits period	15 years