Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 92.7	$ (40.0)
Changes in net unrealized losses on investment securities:
Having no credit losses recognized in the Consolidated Statements of Income	(185.5)	(70.8)
Having credit losses recognized in the Consolidated Statements of Income		(12.4)
Total available-for-sale securities	(185.5)	(83.2)
Pension and postretirement benefits:
Net change in net actuarial loss	0.7	1.2
Total other comprehensive loss, net of tax	(184.8)	(82.0)
Comprehensive loss	$ (92.1)	$ (122.0)