Low Income Housing Tax Credits and Other Tax Credit Investments - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Dec. 31, 2020
Investments In Affordable Housing Projects [Line Items]
Tax credit investment			$ 85,500,000	$ 59,800,000
Renewable Energy Program
Investments In Affordable Housing Projects [Line Items]
Equity investments	$ 3,000,000		3,000,000	1,200,000
Outstanding investment commitments	0		$ 0	$ 1,700,000
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
Tax credit write off		$ 7,600,000
Impairment (recovery) as a result of management's review of tax credit equity investments	$ 1,100,000		$ (300,000)