Other Comprehensive Income - Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Before Tax Amount
Change in net unrealized loss during period	$ (13.9)	$ (6.5)	$ (19.4)
Reclassification adjustment for net loss included in net income	0.1	(0.7)	(0.3)
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity	1.6	1.9	1.9
Change in net unrealized loss on derivatives		(1.1)	(0.2)
Reclassification adjustment for derivative net (gains) losses included in net income	0.0	1.1	0.0
Change in net actuarial loss (gain)	(0.6)	(1.3)	3.9
Total other comprehensive loss	(12.8)	(6.6)	(14.1)
Tax Expense (Benefit)
Change in net unrealized loss during period	(3.6)	(2.7)	(7.6)
Reclassification adjustment for net gains included in net income	0.0	(0.3)	(0.1)
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity	0.4	0.7	0.7
Change in net unrealized loss on derivatives		(0.4)	(0.1)
Reclassification adjustment for derivative net (gains) losses included in net income		0.4
Change in net actuarial loss (gain)	(0.1)	(0.5)	1.6
Total other comprehensive income (loss)	(3.3)	(2.8)	(5.5)
Net of Tax Amount
Change in net unrealized loss during period	(10.3)	(3.8)	(11.8)
Reclassification adjustment for net gains included in net income	0.1	(0.4)	(0.2)
Change in unamortized gain on available-for-sale securities transferred into held-to-maturity	1.2	1.2	1.2
Change in net unrealized loss on derivatives		(0.7)	(0.1)
Reclassification adjustment for derivative net (gains) losses included in net income		0.7
Change in net actuarial loss (gain)	(0.5)	(0.8)	2.3
Other comprehensive income (loss), net of tax	$ (9.5)	$ (3.8)	$ (8.6)