Variable Interest Entities (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016 USD ($) trust	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) trust director	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2012 USD ($)
Variable Interest Entity [Line Items]
Percentage of par value of policy reinsured	15.00%								15.00%
Other revenue									$ 149.6	$ 147.3	$ 88.1
Accumulated losses	$ 2,797.2				$ 3,084.9				2,797.2	3,084.9
Noncontrolling interest in VIE	133.9				454.8				133.9	454.8
Non-controlling interest in consolidated subsidiaries	$ (17.3)	$ (3.1)	$ 21.4	$ 6.3	6.1	$ (16.0)	$ (0.9)	$ (7.3)	7.3	(18.1)	(22.2)
Payments to Acquire Other Investments									$ 39.5	78.0	114.7
HG Global
Variable Interest Entity [Line Items]
Percentage of par value of policy reinsured	15.00%								15.00%
Other revenue									$ 0.0	0.0	$ 0.0
Noncontrolling interest in VIE	$ 16.6				17.1				16.6	17.1
Star & Shield Insurance Exchange [Member]
Variable Interest Entity [Line Items]
Surplus notes					21.0					21.0
Total assets related to consolidated VIE's	9.2				14.2				9.2	14.2
Total liabilities related to consolidated VIE's	4.8				30.3				4.8	$ 30.3
Prospector
Variable Interest Entity [Line Items]
Variable Interest Entity, Qualitative or Quantitative Information, Ownership Percentage										67.60%
Houston General Insurance [Member]
Variable Interest Entity [Line Items]
Surplus notes	5.0								5.0
Total assets related to consolidated VIE's	5.1								5.1
Total liabilities related to consolidated VIE's	5.0								5.0
Accumulated losses	0.5								0.5
Accrued interest on surplus notes	0.6								0.6
Common Stock | BAM
Variable Interest Entity [Line Items]
Issuance of notes												$ 503.0
Star & Shield Insurance Exchange [Member]
Variable Interest Entity [Line Items]
Other revenue									21.0
Noncontrolling interest in VIE	4.4				(16.0)				$ 4.4	$ (16.0)
BAM
Variable Interest Entity [Line Items]
Right to elect directors on board number | director									2
HG Global
Variable Interest Entity [Line Items]
Surplus notes	300.0								$ 300.0			$ 503.0
Noncontrolling interest in VIE	$ 16.6				$ 17.1				$ 16.6	$ 17.1
First Loss Reinsurance Treaty [Member] | HG Global
Variable Interest Entity [Line Items]
Percentage of par value of policy reinsured	15.00%								15.00%
First Loss Reinsurance Treaty [Member] | BAM
Variable Interest Entity [Line Items]
Percentage of Premiums	60.00%								60.00%
First Loss Reinsurance Treaty [Member] | HG Global
Variable Interest Entity [Line Items]
Number of Trust | trust	2								2
Amendment [Member] | Houston General Insurance [Member]
Variable Interest Entity [Line Items]
Surplus notes	$ 5.0								$ 5.0