Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 41.6	$ 36.7
Other comprehensive income (loss), before tax:
Change in net unrealized gains (losses) during period	33.6	(23.7)
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity	(6.0)	0.0
Other Comprehensive Income (Loss), Transfers from Available-for-Sale to Held-to-Maturity Securities, before Tax	0.0	0.5
Defined benefit post-retirement benefits plans:
Change in net actuarial gain	(0.2)	(0.1)
Other comprehensive income (loss), before tax	27.4	(23.3)
Deferred tax (expense) benefit related to other comprehensive income	(7.2)	6.1
Other comprehensive income (loss), net of tax	20.2	(17.2)
Comprehensive income, net of tax	$ 61.8	$ 19.5