Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Statement of Comprehensive Income [Abstract]
Net income	$ 116.0	$ 418.7	$ 358.7
Changes in net unrealized gains (losses) on investment securities:
Having no credit losses recognized in the Consolidated Statements of Income	(822.6)	(243.2)	210.6
Having credit losses recognized in the Consolidated Statements of Income	(3.7)		1.5
Total available-for-sale securities	(826.3)	(243.2)	212.1
Pension and postretirement benefits:
Net actuarial gains (losses) arising in the period	(5.7)	(9.8)	3.1
Amortization recognized as net periodic benefit and postretirement cost	4.3	2.7	4.7
Total pension and postretirement benefits	(1.4)	(7.1)	7.8
Total other comprehensive income (loss), net of tax	(827.7)	(250.3)	219.9
Comprehensive income (loss)	$ (711.7)	$ 168.4	$ 578.6