Variable Interest Entities - Narrative (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Variable Interest Entity [Line Items]
Investment using net asset value per share			$ 45,800,000
Unfunded commitment related to seed capital investments	$ 0	$ 0
Tax Credit Structures
Variable Interest Entity [Line Items]
Carrying amounts	386,100,000	186,500,000
Exposure to loss from liquidity arrangements and obligations to purchase assets	0
Liabilities related to unfunded commitments	215,200,000	56,700,000
Affordable housing tax credits and other tax benefits	57,900,000	48,800,000
Leveraged Leasing Transactions
Variable Interest Entity [Line Items]
Carrying amounts	131,000,000	183,500,000
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	$ 415,300,000	218,900,000
Investment using net asset value per share	10,000,000	0
Other Liabilities
Variable Interest Entity [Line Items]
Investments in community development projects, carrying amount, liabilities	241,100,000	82,900,000
Accounting Standards Update 2015-02
Variable Interest Entity [Line Items]
Fees waived	$ 1,000,000	$ 8,100,000