Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues
Premiums	$ 4,527.4	$ 4,474.5	$ 4,254.4
Net investment income	265.1	281.3	267.4
Net realized and unrealized investment gains (losses):
Net realized gains (losses) from sales and other	17.9	4.9	(2.7)
Net change in fair value of equity securities	13.4	106.5	(43.4)
Impairment losses on investments	(26.3)	(2.0)	(4.6)
Total net realized and unrealized investment gains (losses)	5.0	109.4	(50.7)
Fees and other income	29.8	25.5	23.2
Total revenues	4,827.3	4,890.7	4,494.3
Losses and expenses
Losses and loss adjustment expenses	2,845.2	2,865.5	2,724.6
Amortization of deferred acquisition costs	951.0	926.7	891.8
Interest expense	37.1	37.5	45.1
Loss on repayment of debt	6.2		28.2
Other operating expenses	543.0	538.9	522.1
Total losses and expenses	4,382.5	4,368.6	4,211.8
Income from continuing operations before income taxes	444.8	522.1	282.5
Income tax expense (benefit):
Current	105.9	80.5	46.2
Deferred	(23.1)	12.6	(2.7)
Total income tax expense	82.8	93.1	43.5
Income from continuing operations	362.0	429.0	239.0
Discontinued operations:
Net income	$ 358.7	$ 425.1	$ 391.0
Basic:
Income from continuing operations	$ 9.61	$ 10.72	$ 5.63
Net income per share	$ 9.52	$ 10.62	$ 9.21
Weighted average shares outstanding	37.7	40.0	42.4
Diluted:
Income from continuing operations	$ 9.50	$ 10.56	$ 5.56
Net income per share	$ 9.42	$ 10.46	$ 9.09
Weighted average shares outstanding	38.1	40.6	43.0
Chaucer [Member]
Revenues
Premiums			$ 850.0
Net investment income			54.9
Net realized and unrealized investment gains (losses):
Total revenues		$ 6.7	912.4
Losses and expenses
Losses and loss adjustment expenses			515.5
Amortization of deferred acquisition costs			252.5
Interest expense			3.8
Other operating expenses			115.0
Total losses and expenses			883.0
Discontinued operations:
Sale of business, net of taxes		(1.2)	131.9
Income (loss) from discontinued businesses, net of taxes	$ 0.4	$ 1.6	$ 20.0
Basic:
Sale of business, net of taxes		$ (0.03)	$ 3.11
Income from Chaucer business, net of taxes	$ 0.01	0.04	0.47
Diluted:
Sale of business, net of taxes		(0.03)	3.07
Income (loss) from discontinued businesses, net of taxes	$ 0.01	$ 0.04	$ 0.46
Life [Member]
Discontinued operations:
Income (loss) from discontinued businesses, net of taxes	$ (3.7)	$ (4.3)	$ 0.1
Basic:
Income from Chaucer business, net of taxes	$ (0.10)	$ (0.11)
Diluted:
Income (loss) from discontinued businesses, net of taxes	$ (0.09)	$ (0.11)