Variable Interest Entities - Narrative (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Variable Interest Entity [Line Items]
Unfunded commitment related to seed capital investments	$ 0	$ 0
Tax Credit Structures
Variable Interest Entity [Line Items]
Carrying amounts	700,300,000	549,800,000
Exposure to loss from liquidity arrangements and obligations to purchase assets	0
Liabilities related to unfunded commitments	354,300,000	279,500,000
Affordable housing tax credits and other tax benefits	67,400,000	63,000,000.0
Leveraged Leasing Transactions
Variable Interest Entity [Line Items]
Carrying amounts	42,600,000	56,800,000
Exposure to loss from liquidity arrangements and obligations to purchase assets	$ 0
Leveraged Leasing Transactions | Lower Limit
Variable Interest Entity [Line Items]
Funding of asset's cost via an equity interest	20.00%
Funding of asset's cost via third party non recourse debt holders	70.00%
Leveraged Leasing Transactions | Upper Limit
Variable Interest Entity [Line Items]
Funding of asset's cost via an equity interest	30.00%
Funding of asset's cost via third party non recourse debt holders	80.00%
Other Assets
Variable Interest Entity [Line Items]
Investments in community development projects, carrying amount, assets	$ 749,300,000	602,400,000
Alternative investment	112,000,000.0	29,200,000
Other Liabilities
Variable Interest Entity [Line Items]
Investments in community development projects, carrying amount, liabilities	$ 376,200,000	$ 321,000,000.0