Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Statement of Comprehensive Income [Abstract]
Net Income	$ 214.7	$ 334.6
Other Comprehensive Income (Loss) (Net of Tax and Reclassifications)
Net Unrealized Gains (Losses) on Available for Sale Debt Securities	213.4	180.2
Net Unrealized Gains (Losses) on Cash Flow Hedges	(0.7)	(0.1)
Net Foreign Currency Adjustments	4.6	22.7
Net Pension and Other Postretirement Benefit Adjustments	3.0	0.2
Net Change	220.3	203.0
Comprehensive Income (Loss)	$ 435.0	$ 537.6