Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 36.7	$ 23.2
Other comprehensive income, before tax:
Change in net unrealized (losses) gains during period	(23.7)	9.3
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity	0.5	0.5
Defined benefit post-retirement benefits plans:
Change in net actuarial gain	(0.1)	(0.7)
Other comprehensive income, before tax	(23.3)	9.1
Deferred tax benefit (expense) related to other comprehensive income	6.1	(3.6)
Other comprehensive (loss) income, net of tax	(17.2)	5.5
Comprehensive income, net of tax	$ 19.5	$ 28.7