Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Consolidated Statements of Comprehensive Income [Abstract]
Net income	$ 257.5	$ 202.2	$ 192.1
Investment securities available-for-sale:
Change in net unrealized gains (losses) during the period	144.8	(613.1)	(113.7)
Reclassification adjustment for net losses (gains) included in income	23.5	24.4	(1.1)
Reclassification adjustment for securities transferred from held-to-maturity to available-for-sale	(7.2)	0.2	0.0
OCI, Debt Securities, Available-for-Sale, Transfer To Held-to-Maturity, Gain (Loss), before Adjustment and Tax	(1.3)	(26.1)	20.2
Change in net unrealized loss (gain) on derivatives	0.9	(6.7)	4.2
Defined benefit post-retirement benefit plans:
Other comprehensive income (loss), before tax	160.7	(621.3)	(90.4)
Deferred tax (expense) benefit related to other comprehensive income (loss)	(40.1)	155.2	22.8
Other comprehensive income (loss), net of tax	120.6	(466.1)	(67.6)
Comprehensive income (loss), net of tax	$ 378.1	$ (263.9)	$ 124.5