Variable Interest Entities (Narrative) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Variable Interest Entity [Line Items]
Assets	$ 169,085,700,000		$ 169,085,700,000		$ 170,003,900,000
Liabilities	157,186,900,000		157,186,900,000		158,315,600,000
Exposure to loss from liquidity arrangements and obligations to purchase assets	0		0
Affordable housing tax credits and other tax benefits	22,000,000	$ 20,700,000	68,100,000	$ 59,400,000
Investment company, voluntary fee waived	76,700,000	$ 5,400,000	206,700,000	$ 5,700,000
Contractual obligation to provide financial support	0		0
Other Assets
Variable Interest Entity [Line Items]
Investments in community development projects, carrying amount, assets	949,400,000		949,400,000		919,600,000
Other Assets | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Assets	906,700,000		906,700,000		874,000,000
Other Liabilities
Variable Interest Entity [Line Items]
Investments in community development projects, carrying amount, liabilities	313,500,000		313,500,000		351,600,000
Other Liabilities | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Liabilities	$ 297,700,000		$ 297,700,000		$ 335,900,000