Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Consolidated Statements of Comprehensive Income [Abstract]
Net income	$ 160.2	$ 106.5	$ 95.7
Investment securities available-for-sale:
Change in net unrealized gains (losses) during the period	(13.9)	(6.5)	(19.4)
Reclassification adjustment for net losses (gains) included in income	0.1	(0.7)	(0.3)
Change in unamortized loss on available-for-sale investment securities transferred into held-to-maturity	1.6	1.9	1.9
Change in net unrealized gain (loss) on derivatives	0.0	(1.1)	(0.2)
Reclassification adjustment for derivative net (gains) losses included in net income	0.0	1.1	0.0
Defined benefit post-retirement benefit plans:
Change in net actuarial loss	(0.6)	(1.3)	3.9
Other comprehensive income (loss), before tax	(12.8)	(6.6)	(14.1)
Deferred tax benefit (expense) related to other comprehensive income (loss)	3.3	2.8	5.5
Other comprehensive income (loss), net of tax	(9.5)	(3.8)	(8.6)
Comprehensive income	$ 150.7	$ 102.7	$ 87.1