Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income	$ 48.3	$ 49.1	$ 114.3	$ 128.6
Other comprehensive income, before tax:
Change in net unrealized (losses) gains during period	(4.2)	4.2	69.9	58.0
Reclassification adjustment for net gains included in income	(0.1)	(0.1)	(0.1)	(0.1)
Reclassification adjustment for securities transferred from held-to-maturity to available-for-sale	0.0	0.0	0.0	(6.0)
Unrealized gain on derivatives	0.0	0.0	0.2	0.0
Change in net actuarial loss	0.1	0.2	0.5	0.5
Other comprehensive (loss) income, before tax	(4.4)	3.9	69.5	51.4
Deferred tax expense related to other comprehensive loss (income)	1.1	(1.0)	(18.6)	(13.4)
Other comprehensive (loss) income, net of tax	(3.3)	2.9	50.9	38.0
Comprehensive income, net of tax	$ 45.0	$ 52.0	$ 165.2	$ 166.6