Variable Interest Entities (Narrative) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Variable Interest Entity [Line Items]
Exposure to loss from liquidity arrangements and obligations to purchase assets	$ 0		$ 0
Affordable housing tax credits and other tax benefits	16,400,000	$ 15,600,000	49,200,000	$ 45,200,000
Fair value, investments, entities that calculate net asset value per share, unfunded commitments	0		0		$ 0
Leveraged Leasing Transactions
Variable Interest Entity [Line Items]
VIE carrying amounts	56,900,000		56,900,000		56,800,000
Exposure to loss from liquidity arrangements and obligations to purchase assets	$ 0		$ 0
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	$ 650,400,000		$ 650,400,000		602,400,000
Alternative investment	108,800,000		108,800,000		29,200,000
Other Assets | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
VIE carrying amounts	600,400,000		600,400,000		549,800,000
Other Liabilities
Variable Interest Entity [Line Items]
Investments in community development projects, carrying amount, liabilities	334,800,000		334,800,000		321,000,000.0
Other Liabilities | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Variable interest entity, nonconsolidated, carrying amount, liabilities	$ 311,700,000		$ 311,700,000		$ 279,500,000