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Common Shareholders’ Equity and Non-controlling Interests (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Common Shares Repurchased and Retired [Abstract]
Value of common shares repurchased and retired during the period	$ 4.9	$ 519.1	$ 723.9
Value of each common share repurchased and retired during the period (in dollars per share)	$ 858	$ 877	$ 869
Common Shares Issued [Abstract]
Stock issued during period, shares, new issues	17,917	16,377	25,086
Shares issued to directors (in shares)	2,317	2,272
Dividends, Common Stock [Abstract]
Cash dividends declared and paid	$ 3.2	$ 3.8	$ 4.6
Dividends delared and paid (in dollars per share)	$ 1.00
Noncontrolling interest, ownership percentage by noncontrolling owners
Non-controlling interests	$ (116.8)	$ (124.9)
Common Stock
Common Shares Repurchased and Retired [Abstract]
Common shares repurchased and retired during the period	5,679	592,458	832,725
Restricted Stock
Common Shares Issued [Abstract]
Stock issued during period, shares, new issues	15,600	14,105	17,985
General Board Authorization
Common Shares Repurchased and Retired [Abstract]
Additional authorized repurchase of common shares	635,705
Common shares repurchased and retired during the period		582,493	821,732
Value of common shares repurchased and retired during the period		$ 511.0	$ 713.1
Value of each common share repurchased and retired during the period (in dollars per share)		$ 877	$ 870
Chief Executive Officer | Restricted Stock
Common Shares Issued [Abstract]
Stock issued during period, shares, new issues			5,142
Directors | Restricted Stock
Common Shares Issued [Abstract]
Stock issued during period, shares, new issues			1,959
Repurchased for Employee Benefit Plans
Common Shares Repurchased and Retired [Abstract]
Common shares repurchased and retired during the period		9,965	10,993
HG Global
Dividends, Common Stock [Abstract]
Noncontrolling interest, ownership percentage by noncontrolling owners	3.10%	3.10%
Non-controlling interests	$ 14.4	$ 14.5
NSM Insurance HoldCo, LLC
Dividends, Common Stock [Abstract]
Noncontrolling interest, ownership percentage by noncontrolling owners	3.60%	4.50%
Non-controlling interests	$ 14.9	$ 13.6
Other NSM
Dividends, Common Stock [Abstract]
Noncontrolling interest, ownership percentage by noncontrolling owners	13.40%	13.40%
Non-controlling interests	$ (0.1)	$ 0.3
Kudu
Dividends, Common Stock [Abstract]
Noncontrolling interest, ownership percentage by noncontrolling owners	0.00%	0.00%
Non-controlling interests	$ 2.2	$ 0.0
MediaAlpha
Dividends, Common Stock [Abstract]
Noncontrolling interest, ownership percentage by noncontrolling owners	0.00%	39.00%
Non-controlling interests	$ 0.0	$ 16.2
Other
Dividends, Common Stock [Abstract]
Non-controlling interests	$ (1.5)	$ 1.1
BAM
Dividends, Common Stock [Abstract]
Noncontrolling interest, ownership percentage by noncontrolling owners	100.00%	100.00%
Non-controlling interests	$ (146.7)	$ (170.6)
Total Non-Controlling Excluding Reciprocals
Dividends, Common Stock [Abstract]
Non-controlling interests	$ 29.9	$ 45.7

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