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 Comprehensive Income [Abstract]
Net income	$ 42.5	$ 36.7	$ 93.9	$ 66.0
Other comprehensive (loss) income, before tax:
Change in net unrealized (losses) gains during period	(0.8)	34.9	(67.6)	74.1
Reclassification adjustment for net gains included in income	0.1	0.0	0.1	0.0
Net change in unamortized gains on available-for-sale securities transferred into held-to-maturity	24.8	0.0	24.8	0.0
Unrealized (gain) loss on derivatives	(0.8)	0.2	(0.9)	0.2
Change in net actuarial loss	0.0	0.2	0.0	0.4
Other comprehensive income (loss), before tax	23.3	34.9	(43.6)	73.9
Changes in deferred taxes related to other comprehensive (loss) income	(5.9)	(9.5)	11.1	(19.7)
Other comprehensive (loss) income, net of tax	17.4	25.4	(32.5)	54.2
Comprehensive income, net of tax	$ 59.9	$ 62.1	$ 61.4	$ 120.2