Variable Interest Entities - Narrative (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Variable Interest Entity [Line Items]
Assets	$ 170,003,900,000	$ 136,828,400,000	$ 132,212,500,000
Liabilities	158,315,600,000	125,737,400,000
Investment company, voluntary fee waived	36,400,000
Unfunded commitment related to seed capital investments	0	0
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Seed Capital Investments	0
Tax Credit Structures
Variable Interest Entity [Line Items]
Exposure to loss from liquidity arrangements and obligations to purchase assets	0
Affordable housing tax credits and other tax benefits	78,900,000	67,400,000
Leveraged Leasing Transactions
Variable Interest Entity [Line Items]
Exposure to loss from liquidity arrangements and obligations to purchase assets	0
Leveraged Leasing Transactions | Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Assets	$ 11,400,000	42,600,000
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	$ 919,600,000	749,300,000
Alternative investment		112,000,000.0
Other Assets | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Assets	874,000,000.0	700,300,000
Other Liabilities
Variable Interest Entity [Line Items]
Investments in community development projects, carrying amount, liabilities	351,600,000	376,200,000
Other Liabilities | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liabilities	$ 335,900,000	$ 354,300,000