Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 49.1	$ 41.4	$ 128.6	$ 119.8
Other comprehensive income (loss), before tax:
Change in net unrealized gains (losses) during period	4.2	(6.3)	58.0	(33.9)
Reclassification adjustment for net (gains) losses included in income	(0.1)	0.0	(0.1)	0.0
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity	0.0	0.0	(6.0)	0.0
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity	0.0	0.5	0.0	1.5
Change in net actuarial gain	(0.2)	(0.2)	(0.5)	(0.5)
Other comprehensive income (loss), before tax	3.9	(6.0)	51.4	(32.9)
Deferred tax (expense) benefit related to other comprehensive income	(1.0)	1.6	(13.4)	8.6
Other comprehensive income (loss), net of tax	2.9	(4.4)	38.0	(24.3)
Comprehensive income, net of tax	$ 52.0	$ 37.0	$ 166.6	$ 95.5