Discontinued Chaucer Business (Summary of Discontinued Chaucer Business) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues
Premiums	$ 1,071.7	$ 1,004.4	$ 3,172.4	$ 2,957.0
Net investment income	66.4	62.6	198.0	181.3
Total revenues	1,166.6	1,085.9	3,391.5	3,175.4
Losses and operating expenses
Losses and LAE	676.4	656.5	2,018.5	1,938.2
Amortization of deferred acquisition costs	224.4	211.4	664.7	625.6
Other operating expenses	131.9	124.0	393.0	380.1
Total losses and expenses	1,043.9	1,003.2	3,110.1	2,977.8
Other items:
Interest expense	(11.2)	(11.3)	(33.9)	(33.9)
Net realized and unrealized investment gains (losses)			3.8	21.2
Income tax benefit (expense)	(18.7)	(23.2)	(44.5)	(53.4)
Income (loss) from discontinued Chaucer business, net of income taxes	(3.6)		30.5
Chaucer [Member]
Revenues
Premiums	212.4	222.5	648.2	632.4
Net investment income	12.8	13.4	41.7	37.1
Other income	1.9	2.7	5.4	5.0
Total revenues	227.1	238.6	695.3	674.5
Losses and operating expenses
Losses and LAE	137.0	221.7	364.3	431.7
Amortization of deferred acquisition costs	60.1	63.7	196.7	180.5
Other operating expenses	26.2	27.0	85.7	81.5
Total losses and expenses	223.3	312.4	646.7	693.7
Income (loss) from discontinued Chaucer business before income taxes and other items (previously presented as Chaucer's operating income (loss))	3.8	(73.8)	48.6	(19.2)
Other items:
Interest expense	(1.5)	(0.8)	(2.8)	(2.4)
Net realized and unrealized investment gains (losses)	0.1	0.1	(0.4)	0.7
Expenses related to the anticipated sale of Chaucer	(6.9)		(8.7)
Other income (expense)	(0.2)	0.7	0.4	2.2
Income (loss) from discontinued Chaucer business before income taxes	(4.7)	(73.8)	37.1	(18.7)
Income tax benefit (expense)	1.1	26.6	(6.6)	10.4
Income (loss) from discontinued Chaucer business, net of income taxes	$ (3.6)	$ (47.2)	$ 30.5	$ (8.3)