Other Comprehensive Income/Loss - Schedule of Accumulated Other Comprehensive Income (Loss) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, before Tax		$ 0.2	$ 0.0
Net accumulated other comprehensive gains	$ 1,977.6	1,977.6	1,982.6	$ 2,013.9
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	(0.2)	(0.2)		0.0
Reclassification adjustment for net gains included in income	(0.1)	(0.1)	(0.1)
Other than Temporary Impairment Losses, Investments, Reclassification Adjustment of Noncredit Portion from Held-to-maturity to Available-for-sale Securities, before Tax		0.0	(6.0)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax	0.0	0.0	0.0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(0.1)	(0.1)	(0.1)
Other Comprehensive Loss, Held-to-maturity Security, Reclassification Adjustment from AOCI for Noncredit Portion of OTTI, Sale, after Tax		0.0	(4.4)
Other Comprehensive Loss, Held-to-maturity Security, Reclassification Adjustment from AOCI for Noncredit Portion of OTTI, Sale, Tax		0.0	(1.6)
Net unrealized gains on investment securities available-for-sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net accumulated other comprehensive gains	62.1	62.1		10.6
Net accumulated other comprehensive gains
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net accumulated other comprehensive gains	61.9	61.9	$ 13.4	11.0
Postretirement Health Coverage | Net actuarial gains on defined benefit post-retirement benefit plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net actuarial gains on defined benefit post-retirement benefit plans	$ 0.0	$ 0.0		$ 0.4