Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Statement of Comprehensive Income [Abstract]
Net income	$ 29.3	$ 41.6
Other comprehensive income, before tax:
Change in net unrealized gains during period	39.2	33.6
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity	0.0	(6.0)
Change in net actuarial loss	(0.2)	(0.2)
Other comprehensive income, before tax	39.0	27.4
Deferred tax expense related to other comprehensive income	(10.2)	(7.2)
Other comprehensive income, net of tax	28.8	20.2
Comprehensive income, net of tax	$ 58.1	$ 61.8