Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Consolidated Statements of Comprehensive Income [Abstract]
Net income	$ 192.1	$ 161.2	$ 181.0
Investment securities available-for-sale:
Change in net unrealized (losses) gains during the period	(113.7)	61.8	54.9
Reclassification adjustment for net gains included in income	(1.1)	(0.3)	(0.1)
Reclassification adjustment for securities transferred from held-to-maturity to available-for-sale	0.0	0.0	(6.0)
OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), before Tax	(20.2)	0.0	0.0
Change in net unrealized loss on derivatives	4.2	0.2	0.0
Defined benefit post-retirement benefit plans:
Change in net actuarial loss	0.0	(0.5)	(0.8)
Other comprehensive (loss) income, before tax	(90.4)	61.2	48.0
Deferred tax benefit (expense) related to other comprehensive (loss) income	22.8	(15.6)	(12.4)
Other comprehensive (loss) income, net of tax	(67.6)	45.6	35.6
Comprehensive income, net of tax	$ 124.5	$ 206.8	$ 216.6