Allowance for Credit Losses (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Financing Receivable, Allowance for Credit Loss [Line Items]
Credit loss, excluding accrued interest, expense (reversal)	$ 24.6	$ 10.7	$ (81.5)
Credit loss expense (reversal) and available-for-sale, allowance for credit loss	24.5	12.0	(81.5)
Financing receivable, excluding accrued interest, allowance for credit loss, writeoff (recovery)	3.8	(4.2)	(6.3)
Other financial assets, allowance for credit loss, excluding accrued interest	0.9	0.8	$ 1.0	$ 0.8
Corporate Debt
Financing Receivable, Allowance for Credit Loss [Line Items]
Credit loss, excluding accrued interest, expense (reversal)	$ (0.1)	$ 1.3