Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Statement of Comprehensive Income [Abstract]
Net income	$ 0.2	$ 34.0	$ 127.6	$ 255.2
Changes in net unrealized gains (losses) on investment securities:
Having no credit losses recognized in the Consolidated Statements of Income	(245.1)	(47.8)	(885.9)	(171.4)
Having credit losses recognized in the Consolidated Statements of Income	(2.2)	(0.1)	(5.7)	0.1
Total available-for-sale securities	(247.3)	(47.9)	(891.6)	(171.3)
Pension and postretirement benefits:
Net change in net actuarial loss	1.1	0.6	3.3	2.0
Total other comprehensive loss, net of tax	(246.2)	(47.3)	(888.3)	(169.3)
Comprehensive income (loss)	$ (246.0)	$ (13.3)	$ (760.7)	$ 85.9