Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Consolidated Statements of Comprehensive Income [Abstract]
Net income	$ 161.2	$ 181.0	$ 160.2
Investment securities available-for-sale:
Change in net unrealized gains (losses) during the period	61.8	54.9	(13.9)
Reclassification adjustment for net (gains) losses included in income	(0.3)	(0.1)	0.1
Reclassification adjustment for securities transferred from held-to-maturity to available-for-sale	0.0	(6.0)	0.0
Change in unamortized loss on available-for-sale investment securities transferred into held-to-maturity	0.0	0.0	1.6
Change in net unrealized loss on derivatives	0.2	0.0	0.0
Defined benefit post-retirement benefit plans:
Change in net actuarial loss	(0.5)	(0.8)	(0.6)
Other comprehensive income (loss), before tax	61.2	48.0	(12.8)
Deferred tax (expense) benefit related to other comprehensive income (loss)	(15.6)	(12.4)	3.3
Other comprehensive income (loss), net of tax	45.6	35.6	(9.5)
Comprehensive income, net of tax	$ 206.8	$ 216.6	$ 150.7