Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Statement of Comprehensive Income [Abstract]
Net income	$ 35.3	$ 116.0	$ 422.8
Changes in net unrealized gains (losses) on investment securities:
Having no credit losses recognized in the Consolidated Statements of Income	175.0	(822.6)	(243.2)
Having credit losses recognized in the Consolidated Statements of Income	3.6	(3.7)
Total available-for-sale securities	178.6	(826.3)	(243.2)
Pension and postretirement benefits:
Net actuarial gains (losses) arising in the period	3.1	(5.7)	(9.8)
Amortization recognized as net periodic benefit and postretirement cost	6.2	4.3	2.7
Total pension and postretirement benefits	9.3	(1.4)	(7.1)
Long-duration insurance contracts:
Net change in market risk	(3.6)	17.0	4.2
Total other comprehensive income (loss), net of tax	184.3	(810.7)	(246.1)
Comprehensive income (loss)	$ 219.6	$ (694.7)	$ 176.7