Allowance for Credit Losses (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Jun. 30, 2023	Dec. 31, 2022	Jun. 30, 2022	Dec. 31, 2021
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Line Items]
Provision for (release of) credit losses	$ 14.6	$ 1.4	$ 14.1	$ 6.2
Net recoveries (charge-offs)	(0.3)	4.5	$ 2.6	(4.2)
Time period used for projecting future conditions to estimate credit losses			2 years
Other financial assets, allowance for credit loss	1.1	0.7	$ 1.1	0.7	$ 1.0	$ 0.8	$ 1.1	$ 1.0
Corporate Debt
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Line Items]
Provision for (release of) credit losses	$ (0.6)	$ (0.9)	$ (0.6)	$ 0.8