Variable Interest Entities - Narrative (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Unfunded commitment related to seed capital investments	$ 0	$ 0
Tax Credit Structures
Variable Interest Entity [Line Items]
Carrying amounts	549,800,000	386,100,000
Exposure to loss from liquidity arrangements and obligations to purchase assets	0
Liabilities related to unfunded commitments	279,500,000	215,200,000
Affordable housing tax credits and other tax benefits	63,000,000	57,900,000
Leveraged Leasing Transactions
Variable Interest Entity [Line Items]
Carrying amounts	56,800,000	131,000,000
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	$ 602,400,000	415,300,000
Alternative investment	29,200,000	10,000,000
Other Liabilities
Variable Interest Entity [Line Items]
Investments in community development projects, carrying amount, liabilities	$ 321,000,000	241,100,000
Accounting Standards Update 2015-02
Variable Interest Entity [Line Items]
Fees waived		$ 1,000,000