Variable Interest Entities (Narrative) (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Variable Interest Entity [Line Items]
Exposure to loss from liquidity arrangements and obligations to purchase assets	$ 0		$ 0
Assets	151,432,900,000		151,432,900,000		$ 136,828,400,000
Liabilities	140,050,700,000		140,050,700,000		125,737,400,000
Affordable housing tax credits and other tax benefits	22,400,000	$ 16,400,000	38,700,000	$ 32,700,000
Investment company, voluntary fee waived	300,000	$ 0	300,000	$ 0
Contractual obligation to provide financial support	0		0
Fair value, investments, entities that calculate net asset value per share, unfunded commitments	74,600,000		74,600,000		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	$ 30,300,000		$ 30,300,000		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	$ 725,800,000		$ 725,800,000		749,300,000
Alternative investment	55,300,000		55,300,000		112,000,000.0
Other Assets | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Assets	678,800,000		678,800,000		700,300,000
Other Liabilities
Variable Interest Entity [Line Items]
Investments in community development projects, carrying amount, liabilities	319,000,000.0		319,000,000.0		376,200,000
Other Liabilities | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liabilities	$ 301,700,000		$ 301,700,000		$ 354,300,000