Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Consolidated Statements of Comprehensive Income [Abstract]
Net income	$ 181.0	$ 160.2	$ 106.5
Investment securities available-for-sale:
Change in net unrealized gains (losses) during the period	54.9	(13.9)	(6.5)
Reclassification adjustment for net (gains) losses included in income	(0.1)	0.1	(0.7)
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity	(6.0)	0.0	0.0
Other Comprehensive Income (Loss), Transfers From Available-For-Sale To Held-To-Maturity Securities, Before Tax	281.1	1.6	1.9
Change in net unrealized loss on derivatives	0.0	0.0	(1.1)
Reclassification adjustment for derivative net (gains) losses included in net income	0.0	0.0	1.1
Defined benefit post-retirement benefit plans:
Change in net actuarial loss	(0.8)	(0.6)	(1.3)
Other comprehensive income (loss), before tax	48.0	(12.8)	(6.6)
Deferred tax benefit (expense) related to other comprehensive income (loss)	(12.4)	3.3	2.8
Other comprehensive income (loss), net of tax	35.6	(9.5)	(3.8)
Comprehensive income	$ 216.6	$ 150.7	$ 102.7