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Municipal Bond Guarantee (Details) $ in Millions	1 Months Ended	9 Months Ended		12 Months Ended
	Apr. 30, 2018 USD ($)	Sep. 30, 2018 USD ($) yr Contract	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) Contract	Dec. 31, 2012 USD ($)	Jul. 31, 2012 USD ($)
Guarantor Obligations [Line Items]
Repayments of debt		$ 11.2	$ 364.6
Interest paid		5.3	$ 0.5
Total gross unearned insurance premiums		156.2		$ 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		7,184		6,371
Remaining weighted average contract period outstanding (in years)		10 years 10 months		10 years 11 months
Municipal bond outstanding principal		$ 47,259.1		$ 42,090.6
Municipal bond outstanding interest		23,707.4		21,057.1
Municipal bond debt service outstanding		70,966.5		63,147.7
October 1, 2018 - December 31, 2018		3.3
2019		13.1
2020		12.6
2021		12.1
2022		11.5
2023 and thereafter		103.6
Total gross unearned insurance premiums		156.2
HG Global
Guarantor Obligations [Line Items]
Collateral held in supplement trust		741.7		715.1
Interest receivable		$ 143.2		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%		3.78%
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 spread on variable rate		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		$ 3.3
Second Quarter, year two | BAM
Guarantor Obligations [Line Items]
2019		3.3
Third Quarter, year two | BAM
Guarantor Obligations [Line Items]
2019		3.3
Fourth Quarter, year two | BAM
Guarantor Obligations [Line Items]
2019		3.2
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
BAM | Fidus Re Ltd.
Guarantor Obligations [Line Items]
Losses above retention, percentage of liability	90.00%
Fidus Re Ltd. | BAM
Guarantor Obligations [Line Items]
Collateralized financial guarantee, term	12 years
Aggregate losses on ceded business	$ 165.0
Losses above retention, percentage of liability	10.00%
Minimum | Fidus Re Ltd. | BAM
Guarantor Obligations [Line Items]
Aggregate losses on ceded business	$ 165.0
Maximum | Fidus Re Ltd. | BAM
Guarantor Obligations [Line Items]
Aggregate losses on ceded business		$ 276.0

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