Other Comprehensive Income - Schedule of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Equity [Abstract]
Other Comprehensive Income (Loss), Tax	$ 12.4	$ 12.4	$ (3.3)	$ (2.8)
Other Comprehensive Income (Loss), Transfers From Available-For-Sale To Held-To-Maturity Securities, Net Of Tax			1.2	1.2
Other Comprehensive Income (Loss), Net of Tax	35.6		(9.5)
Before Tax Amount
Change in net unrealized gain during period		54.9	(13.9)	(6.5)
Reclassification adjustment for net gain included in net income		(0.1)	0.1	(0.7)
Change in net unrealized loss on derivatives				(1.1)
Reclassification adjustment for derivative net (gains) losses included in net income		0.0	0.0	1.1
Change in net actuarial gains		(0.8)	(0.6)	(1.3)
Total other comprehensive income				(6.6)
Tax Expense (Benefit)
Change in net unrealized loss during period		14.1	(3.6)	(2.7)
Reclassification adjustment for net loss included in net income		0.0	0.0	(0.3)
Change in net unrealized loss on derivatives				(0.4)
Change in net actuarial gains		(0.1)	(0.1)	(0.5)
Total other comprehensive income (loss)				(2.8)
Net of Tax Amount
Change in net unrealized loss during period		40.8	(10.3)	(3.8)
Reclassification adjustment for net gain included in net income		(0.1)	0.1	(0.4)
Change in net unrealized loss on derivatives				(0.7)
Change in net actuarial loss		(0.7)	(0.5)	(0.8)
Other comprehensive income (loss), net of tax		35.6	(9.5)	(3.8)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax				0.4
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax				0.7
Other comprehensive income (loss), before tax	$ 48.0	48.0	(12.8)	(6.6)
Other Comprehensive Income (Loss), Transfers From Available-For-Sale To Held-To-Maturity Securities, Before Tax		281.1	1.6	1.9
Other Comprehensive Income (Loss), Transfers From Available-For-Sale To Held-To-Maturity Securities, Tax			0.4	0.7
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity		(6.0)	$ 0.0	$ 0.0
Other Comprehensive Income (Loss), Transfers from Held-to-maturity to Available-for-Sale Securities, Tax		(1.6)
Other Comprehensive Income (Loss), Transfers from Held-to-maturity to Available-for-Sale Securities, Net of Tax		$ (4.4)