Other Comprehensive Income - Schedule of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Equity [Abstract]
Other Comprehensive Income (Loss), Tax	$ 15.6	$ 15.6	$ 12.4	$ (3.3)
Other Comprehensive Income (Loss), Transfers From Available-For-Sale To Held-To-Maturity Securities, Net Of Tax				1.2
Other Comprehensive Income (Loss), Net of Tax	45.6		35.6
Before Tax Amount
Change in net unrealized gains during period		61.8	54.9	(13.9)
Reclassification adjustment for net gains included in net income		(0.3)	(0.1)	0.1
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Investment Transferred from Available-for-sale to Equity Method, Tax			(1.6)
Change in net unrealized loss on derivatives		0.2
Change in net actuarial gains		(0.5)	(0.8)	(0.6)
Total other comprehensive income				(12.8)
Tax Expense (Benefit)
Change in net unrealized gains during period		15.8	14.1	(3.6)
Reclassification adjustment for net gains included in net income		(0.1)	0.0	0.0
Change in net unrealized loss on derivatives		0.0
Change in net actuarial gains		(0.1)	(0.1)	(0.1)
Total other comprehensive income (loss)				(3.3)
Net of Tax Amount
Change in net unrealized loss during period		46.0	40.8	(10.3)
Reclassification adjustment for net loss included in net income		(0.2)	(0.1)	0.1
Change in net unrealized loss on derivatives		0.2
Change in net actuarial loss		(0.4)	(0.7)	(0.5)
Other comprehensive income (loss), net of tax		45.6	35.6	(9.5)
Other comprehensive income (loss), before tax	$ 61.2	61.2	48.0	(12.8)
Change in unamortized loss on available-for-sale investment securities transferred into held-to-maturity		0.0	0.0	1.6
Other Comprehensive Income (Loss), Transfers From Available-For-Sale To Held-To-Maturity Securities, Tax				0.4
Reclassification adjustment for securities transferred from held-to-maturity to available-for-sale		0.0	(6.0)	0.0
Other Comprehensive Income (Loss), Transfers from Held-to-maturity to Available-for-Sale Securities, Net of Tax			(4.4)
Change in net unrealized loss on derivatives		$ (0.2)	$ 0.0	$ 0.0