Common Shareholders’ Equity and Non-controlling Interests (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Common Shares Repurchased and Retired [Abstract]
Value of common shares repurchased and retired during the period	$ 519.4	$ 723.9	$ 887.2
Value of each common share repurchased and retired during the period (in dollars per share)	$ 876.69	$ 869	$ 802
Common Shares Issued [Abstract]
Stock issued during period, shares, new issues	16,377	25,086	47,030
Shares issued to directors (in shares)	2,272	1,959	1,735
Dividends, Common Stock [Abstract]
Cash dividends declared and paid	$ 3.8	$ 4.6	$ 5.4
Dividends delared and paid (in dollars per share)	$ 1.00
Non-controlling interests	$ (124.9)	$ (131.7)
Common Stock
Common Shares Repurchased and Retired [Abstract]
Common shares repurchased and retired during the period	592,458	832,725	1,106,145
Restricted Stock
Common Shares Issued [Abstract]
Stock issued during period, shares, new issues	14,105	17,985	25,365
Performance Shares
Common Shares Issued [Abstract]
Stock issued during period, shares, new issues			5,000
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	1,098,123
Value of common shares repurchased and retired during the period	$ 511.0	$ 713.1	$ 881.4
Value of each common share repurchased and retired during the period (in dollars per share)	$ 877	$ 870	$ 802.63
Chief Executive Officer \| Restricted Stock
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	9,965	10,993	8,022
HG Global
Dividends, Common Stock [Abstract]
Noncontrolling interest, ownership percentage by noncontrolling owners	3.10%	3.10%
Non-controlling interests	$ 14.5	$ 15.9
NSM Insurance HoldCo, LLC
Dividends, Common Stock [Abstract]
Noncontrolling interest, ownership percentage by noncontrolling owners	4.50%	0.00%
Non-controlling interests	$ 13.6	$ 0.0
MediaAlpha
Dividends, Common Stock [Abstract]
Noncontrolling interest, ownership percentage by noncontrolling owners	39.00%	35.70%
Non-controlling interests	$ 16.2	$ 13.1
Buzzmove
Dividends, Common Stock [Abstract]
Noncontrolling interest, ownership percentage by noncontrolling owners	22.90%	22.90%
Non-controlling interests	$ 1.1	$ 2.5
Other
Dividends, Common Stock [Abstract]
Noncontrolling interest, ownership percentage by noncontrolling owners	13.40%
Non-controlling interests	$ 0.3
BAM
Dividends, Common Stock [Abstract]
Noncontrolling interest, ownership percentage by noncontrolling owners	100.00%	100.00%
Non-controlling interests	$ (170.6)	$ (163.2)
Total Non-Controlling Excluding Reciprocals
Dividends, Common Stock [Abstract]
Non-controlling interests	$ 45.7	$ 31.5

Aggregate dividends paid during the period for each share of common stock outstanding.

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