Discontinued Operations Chaucer Business (Summary of Discontinued Chaucer Business) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Revenues
Premiums	$ 1,095.1	$ 1,042.5
Net investment income	70.2	66.0
Total revenues	1,219.5	1,091.5
Losses and operating expenses
Losses and loss adjustment expenses	699.6	670.9
Amortization of deferred acquisition costs	229.5	218.7
Other operating expenses	132.4	131.5
Total losses and expenses	1,070.9	1,032.4
Other items:
Interest expense	(9.4)	(11.3)
Net realized and unrealized investment losses	48.4	(23.4)
Chaucer [Member]
Revenues
Premiums		221.1
Net investment income		16.4
Other income		1.9
Total revenues	2.9	239.4
Losses and operating expenses
Losses and loss adjustment expenses		115.0
Amortization of deferred acquisition costs		69.3
Other operating expenses		32.2
Total losses and expenses		216.5
Income from Chaucer business before income taxes and other items (previously presented as Chaucer's operating income)		22.9
Other items:
Interest expense		(1.1)
Net realized and unrealized investment losses		(0.5)
Other income		0.2
Income from Chaucer business before income taxes		21.5
Income tax expense		(4.2)
Income from Chaucer business, net of income taxes	$ (0.3)	$ 17.3