Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 128.5	$ 115.2	$ 221.2	$ 75.2
Changes in net unrealized gains (losses) on investment securities:
Having no credit losses recognized in the Consolidated Statements of Income	61.9	241.7	(123.6)	170.9
Having credit losses recognized in the Consolidated Statements of Income	0.2	10.0	0.2	(2.4)
Total available-for-sale securities	62.1	251.7	(123.4)	168.5
Pension and postretirement benefits:
Net change in net actuarial loss	0.7	1.2	1.4	2.4
Total other comprehensive income (loss), net of tax	62.8	252.9	(122.0)	170.9
Comprehensive income	$ 191.3	$ 368.1	$ 99.2	$ 246.1