Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Consolidated Statements of Comprehensive Income [Abstract]
Net income	$ 202.2	$ 192.1	$ 161.2
Investment securities available-for-sale:
Change in net unrealized (losses) gains during the period	(613.1)	(113.7)	61.8
Reclassification adjustment for net losses (gains) included in income	24.4	(1.1)	(0.3)
Reclassification adjustment for securities transferred from held-to-maturity to available-for-sale	0.2	0.0	0.0
OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), before Tax	26.1	(20.2)	0.0
Change in net unrealized (gain) loss on derivatives	(6.7)	4.2	0.2
Defined benefit post-retirement benefit plans:
Change in net actuarial loss	0.0	0.0	(0.5)
Other comprehensive (loss) income, before tax	(621.3)	(90.4)	61.2
Deferred tax benefit (expense) related to other comprehensive (loss) income	155.2	22.8	(15.6)
Other comprehensive (loss) income, net of tax	(466.1)	(67.6)	45.6
Comprehensive (loss) income, net of tax	$ (263.9)	$ 124.5	$ 206.8