Common Shareholders’ Equity and Non-controlling Interests (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Common Shares Repurchased and Retired [Abstract]
Value of common shares repurchased and retired during the period	$ 723.9	$ 887.2	$ 284.2
Value of each common share repurchased and retired during the period (in dollars per share)	$ 869	$ 802	$ 733
Common Shares Issued [Abstract]
Stock issued during period, shares, new issues	25,086	47,030	25,016
Shares issued to directors (in shares)	1,959	1,735	1,376
Stock issued, in satisfaction of performance shares		5,000
Dividends, Common Stock [Abstract]
Cash dividends declared and paid	$ 4.6	$ 5.4	$ 6.0
Dividends declared and paid (in dollars per share)	$ 1.00	$ 1.00	$ 1.00
Non-controlling Equity	$ (131.7)	$ 133.3
Total Non-Controlling Excluding Reciprocals
Dividends, Common Stock [Abstract]
Non-controlling Equity	$ 31.5	$ 35.2
Restricted Stock
Common Shares Issued [Abstract]
Stock issued during period, shares, new issues	17,985	25,365	23,640
Common Stock
Common Shares Repurchased and Retired [Abstract]
Common shares repurchased and retired during the period	832,725	1,106,145	387,495
General Board Authorization
Common Shares Repurchased and Retired [Abstract]
Additional authorized repurchase of common shares	643,130
Common shares repurchased and retired during the period	821,732	1,098,123	361,839
Value of common shares repurchased and retired during the period	$ 713.1	$ 881.4	$ 267.4
Value of each common share repurchased and retired during the period (in dollars per share)	$ 870	$ 803	$ 739
Chief Executive Officer
Common Shares Issued [Abstract]
Stock issued during period, shares, new issues	5,142	14,930
Repurchased for Employee Benefit Plans
Common Shares Repurchased and Retired [Abstract]
Common shares repurchased and retired during the period	10,993	8,022	10,802
OneBeacon Ltd.
Dividends, Common Stock [Abstract]
Non-controlling Percentage	0.00%	23.90%
Non-controlling Equity	$ 0.0	$ 244.6
HG Global
Dividends, Common Stock [Abstract]
Non-controlling Percentage	3.10%	3.10%
Non-controlling Equity	$ 15.9	$ 16.6
MediaAlpha
Dividends, Common Stock [Abstract]
Non-controlling Percentage	35.70%	40.00%
Non-controlling Equity	$ 13.1	$ 11.7
Dewar
Dividends, Common Stock [Abstract]
Non-controlling Percentage	0.00%	18.80%
Non-controlling Equity	$ 0.0	$ 3.9
Buzzmove
Dividends, Common Stock [Abstract]
Non-controlling Percentage	22.90%	29.10%
Non-controlling Equity	$ 2.5	$ 3.0
BAM
Dividends, Common Stock [Abstract]
Non-controlling Percentage	100.00%	100.00%
Non-controlling Equity	$ (163.2)	$ (150.9)
Star & Shield Insurance Exchange
Dividends, Common Stock [Abstract]
Non-controlling Percentage	0.00%	100.00%
Non-controlling Equity	$ 0.0	$ 4.4
Total mutuals and reciprocals
Dividends, Common Stock [Abstract]
Non-controlling Equity	$ (163.2)	$ (146.5)