Municipal Bond Guarantee (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) Contract yr	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) Contract	Dec. 31, 2012 USD ($)	Jul. 31, 2012 USD ($)
Guarantor Obligations [Line Items]
Repayments of debt	$ 2.3	$ 1.2
Interest paid	0.3	$ 0.2
Total gross unearned insurance premiums	140.2		$ 136.8
HG Global
Guarantor Obligations [Line Items]
Partners capital contribution				$ 594.5
Surplus notes	$ 499.0				$ 503.0
Assets held-in-trust			$ 603.0
HG Global | Preferred Stock
Guarantor Obligations [Line Items]
Ownership interest (as a percent)	96.90%		96.90%
HG Global | Common Stock
Guarantor Obligations [Line Items]
Ownership interest (as a percent)	88.40%		88.40%
BAM
Guarantor Obligations [Line Items]
Percentage of Risk Premium Ceded	60.00%
Year of U.S. Treasury rate | yr	1
Contracts outstanding | Contract	6,541		6,371
Remaining weighted average contract period outstanding (in years)	10 years 10 months		10 years 11 months
Municipal bond outstanding principal	$ 43,139.2		$ 42,090.6
Municipal bond outstanding interest	21,420.6		21,057.1
Municipal bond debt service outstanding	64,559.8		63,147.7
April 1, 2018 - December 31, 2018	8.9
2019	11.6
2020	11.1
2021	10.6
2022	10.1
2023 and thereafter	87.9
Total gross unearned insurance premiums	140.2
HG Global
Guarantor Obligations [Line Items]
Collateral held in supplement trust	721.9		715.1
Interest receivable	$ 131.7		126.0
HG Re
Guarantor Obligations [Line Items]
Percentage of par value of policy reinsured	15.00%
BAM
Guarantor Obligations [Line Items]
Surplus notes	$ 499.0		$ 499.0
BAM Management
Guarantor Obligations [Line Items]
Partners capital contribution				$ 14.5
Surplus Note | BAM
Guarantor Obligations [Line Items]
Derivative, fixed interest rate (as a percent)	4.60%		3.54%
Surplus Note | BAM
Guarantor Obligations [Line Items]
Repayments of debt			$ 4.0
Interest paid			1.0
Combination One | Surplus Note | BAM
Guarantor Obligations [Line Items]
Number of years of change in interest rate | yr	5
Combination Two | Surplus Note | BAM
Guarantor Obligations [Line Items]
Number of years of change in interest rate | yr	3
US Treasury Rate | Surplus Note | BAM
Guarantor Obligations [Line Items]
Basis points	3.00%
Fixed interest rate | Surplus Note | BAM
Guarantor Obligations [Line Items]
Derivative, fixed interest rate (as a percent)	8.00%
First Quarter, year two | BAM
Guarantor Obligations [Line Items]
2019	$ 2.9
Second Quarter, year two | BAM
Guarantor Obligations [Line Items]
2019	2.9
Third Quarter, year two | BAM
Guarantor Obligations [Line Items]
2019	2.9
Fourth Quarter, year two | BAM
Guarantor Obligations [Line Items]
2019	$ 2.9
Series A BAM Surplus Note | HG Global
Guarantor Obligations [Line Items]
Surplus notes			203.0
Series B BAM Surplus Note | HG Global
Guarantor Obligations [Line Items]
Surplus notes			$ 300.0