Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Statement of Comprehensive Income [Abstract]
Net income	$ 47.1	$ 48.3	$ 141.0	$ 114.3
Other comprehensive (loss) income, before tax:
Change in net unrealized (losses) gains during period	(13.4)	(4.2)	(81.0)	69.9
Reclassification adjustment for net gain included in income	(0.3)	(0.1)	(0.2)	(0.1)
Net change in unamortized gains on available-for-sale securities transferred into held-to-maturity	(2.5)	0.0	22.3	0.0
Unrealized loss on derivatives	3.2	0.0	2.3	0.2
Change in net actuarial loss	0.0	0.1	0.0	0.5
Other comprehensive income (loss), before tax	(13.0)	(4.4)	(56.6)	69.5
Changes in deferred taxes related to other comprehensive income (loss)	3.2	1.1	14.3	(18.6)
Other comprehensive (loss) income, net of tax	(9.8)	(3.3)	(42.3)	50.9
Comprehensive income, net of tax	$ 37.3	$ 45.0	$ 98.7	$ 165.2