Investments (Unrealized Gains and Losses on Available-For-Sale and Other Securities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Provision for deferred income taxes	$ 68.1	$ 83.0	$ (28.5)
Net Unrealized Appreciation (Depreciation) on Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Net appreciation (depreciation), beginning of year	216.0	(27.2)	205.4
Net appreciation (depreciation) on available-for-sale securities	280.2	324.7	(203.7)
Provision for deferred income taxes	(68.1)	(83.0)	33.6
Amount realized with sale of Chaucer business		0.1	19.1
Cumulative effect adjustment for ASU 2017-08, 2016-01 and 2018-02, net of tax		1.5	(81.6)
Total adjustment	212.1	243.2	(232.6)
Net appreciation (depreciation), end of year	428.1	216.0	(27.2)
Fixed Maturities [Member] | Net Unrealized Appreciation (Depreciation) on Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Net appreciation (depreciation), beginning of year	216.0	(27.2)	110.1
Net appreciation (depreciation) on available-for-sale securities	280.2	324.7	(203.7)
Provision for deferred income taxes	(68.1)	(83.0)	33.6
Amount realized with sale of Chaucer business			19.1
Cumulative effect adjustment for ASU 2017-08, 2016-01 and 2018-02, net of tax		1.5	13.7
Total adjustment	212.1	243.2	(137.3)
Net appreciation (depreciation), end of year	$ 428.1	$ 216.0	(27.2)
Equity Securities and Other [Member] | Net Unrealized Appreciation (Depreciation) on Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Net appreciation (depreciation), beginning of year			95.3
Cumulative effect adjustment for ASU 2017-08, 2016-01 and 2018-02, net of tax			(95.3)
Total adjustment			$ (95.3)