Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 104.8	$ 92.7
Changes in net unrealized losses on investment securities:
Having no credit losses recognized in the Consolidated Statements of Income	(379.4)	(185.5)
Having credit losses recognized in the Consolidated Statements of Income	(0.5)
Total available-for-sale securities	(379.9)	(185.5)
Pension and postretirement benefits:
Net change in net actuarial loss	1.1	0.7
Total other comprehensive loss, net of tax	(378.8)	(184.8)
Comprehensive loss	$ (274.0)	$ (92.1)