Variable Interest Entities (Narrative) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Variable Interest Entity [Line Items]
Exposure to loss from liquidity arrangements and obligations to purchase assets	$ 0		$ 0
Assets	152,082,000,000.0		152,082,000,000.0		$ 136,828,400,000
Liabilities	140,502,900,000		140,502,900,000		125,737,400,000
Affordable housing tax credits and other tax benefits	20,700,000	$ 16,400,000	59,400,000	$ 49,200,000
Contractual obligation to provide financial support	0		0
Fair value, investments, entities that calculate net asset value per share, unfunded commitments	73,300,000		73,300,000		0
Leveraged Leasing Transactions
Variable Interest Entity [Line Items]
Exposure to loss from liquidity arrangements and obligations to purchase assets	0		0
Leveraged Leasing Transactions | Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Assets	$ 30,100,000		$ 30,100,000		42,600,000
Leveraged Leasing Transactions | Lower Limit
Variable Interest Entity [Line Items]
Funding of assets cost via equity interest	20.00%		20.00%
Funding of asset's cost via third party non recourse debt holders	70.00%		70.00%
Leveraged Leasing Transactions | Upper Limit
Variable Interest Entity [Line Items]
Funding of assets cost via equity interest	30.00%		30.00%
Funding of asset's cost via third party non recourse debt holders	80.00%		80.00%
Other Assets
Variable Interest Entity [Line Items]
Investments in community development projects, carrying amount, assets	$ 761,300,000		$ 761,300,000		749,300,000
Alternative investment	1,700,000		1,700,000		112,000,000.0
Other Assets | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Assets	715,500,000		715,500,000		700,300,000
Other Liabilities
Variable Interest Entity [Line Items]
Investments in community development projects, carrying amount, liabilities	300,000,000.0		300,000,000.0		376,200,000
Other Liabilities | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liabilities	$ 283,500,000		$ 283,500,000		$ 354,300,000