Debt Securities (Sales of Debt Securities) (Narrative) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Investments, Debt and Equity Securities [Abstract]
Realized losses on sales of debt securities	$ 3.5	$ 7.3	$ 13.5	$ 11.5
Realized gains on sales of debt securities	0.7	0.5	6.2	4.0
Proceeds from sales of debt securities	$ 187.5	$ 161.2	$ 903.0	$ 457.4