Discontinued Operations (Summary of Discontinued Chaucer Business) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues
Net premiums earned									$ 4,527.4	$ 4,474.5	$ 4,254.4
Net investment income									265.1	281.3	267.4
Total revenues	$ 1,297.0	$ 1,249.8	$ 1,224.3	$ 1,056.2	$ 1,257.9	$ 1,214.7	$ 1,198.6	$ 1,219.5	4,827.3	4,890.7	4,494.3
Losses and operating expenses
Losses and LAE									2,845.2	2,865.5	2,724.6
Amortization of deferred acquisition costs									951.0	926.7	891.8
Other expenses									543.0	538.9	522.1
Total losses and expenses									4,382.5	4,368.6	4,211.8
Other items:
Interest expense									(37.1)	(37.5)	(45.1)
Net realized and unrealized investment losses									5.0	109.4	(50.7)
Chaucer [Member]
Revenues
Net premiums earned											850.0
Net investment income											54.9
Other income											7.5
Total revenues										6.7	912.4
Losses and operating expenses
Losses and LAE											515.5
Amortization of deferred acquisition costs											252.5
Other expenses											115.0
Total losses and expenses											883.0
Income from Chaucer business before income taxes and other items (previously presented as Chaucer's operating income)											29.4
Other items:
Interest expense											(3.8)
Net realized and unrealized investment losses											(1.3)
Other income											0.4
Income from Chaucer business before income taxes											24.7
Income tax expense											(4.7)
Income from Chaucer business, net of taxes									$ 0.4	$ 1.6	$ 20.0