Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Statement of Comprehensive Income [Abstract]
Net income	$ 128.2	$ 115.5
Changes in net unrealized gains (losses) on investment securities:
Having no credit losses recognized in the Consolidated Statements of Income	111.3	(33.9)
Having credit losses recognized in the Consolidated Statements of Income	(0.1)	(0.2)
Total available-for-sale securities	111.2	(34.1)
Pension and postretirement benefits:
Net change in net actuarial loss	1.4	1.4
Long-duration insurance contracts:
Net change in market risk	(1.0)	1.0
Total other comprehensive income (loss), net of tax	111.6	(31.7)
Comprehensive income	$ 239.8	$ 83.8