The required period of service for an employee to become vested in the plan.

No definition available.

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- Definition

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

+ References

No definition available.

+ Details

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- Definition

Maximum amount of discretionary contributions that can made by an employer to a defined contribution plan, as a percentage of salary.

+ References

No definition available.

+ Details

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- Definition

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan, first level.

+ References

No definition available.

+ Details

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- Definition

Percentage employer matches of the employee's percentage contribution matched, first level.

+ References

No definition available.

+ Details

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- Definition

Percentage employer matches of the employee's percentage contribution matched, second level.

+ References

No definition available.

+ Details

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- Definition

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan, second level.

+ References

No definition available.

+ Details

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- Definition

Amount of cost for matching contributions in the defined contribution plan.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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- Definition

Amount of cost for variable contributions in the defined contribution plan.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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- Definition

The amount of IRS annual covered compensation limits applicable under the plan.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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- Definition

Percentage of total premium charges incurred by the employee.

+ References

No definition available.

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of total premium charges incurred by the employer.

+ References

No definition available.

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Definition

Amount of discretionary contributions made by an employer to a defined contribution plan.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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- Definition

Amount of cost (reversal of cost) for pension benefits. Excludes other postretirement benefits.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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- Details

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- Details

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- Details

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- Details

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- Details

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- Details

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Employee benefit plans and compensation plans - Defined contribution plans - Non U.S and U.S. (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined contribution plans
Benefit plan expenses recognized		$ 8.3	$ 7.6	$ 4.9
SIHC 401(k) Plan
Defined contribution plans
Employer match, first level, percent		100.00%
Percentage of salary matched, first level, percent		2.00%
Employer match, second level, percent		50.00%
Percentage of salary matched, second level, percent		4.00%
Matching contribution expense		$ 0.8	0.8	1.3
Variable contribution as percentage of salary		7.00%
IRS annual covered compensation limit		$ 0.3
Variable contribution expense				0.3
IMG 401(k) Plan
Defined contribution plans
Employer match, first level, percent		100.00%
Percentage of salary matched, first level, percent		1.00%
Employer match, second level, percent		50.00%
Matching contribution expense	$ 0.3	$ 0.7	0.6
Armada 401(k) Plan
Defined contribution plans
Discretionary matching contributions	$ 0.1	$ 0.2	0.2
Minimum \| IMG 401(k) Plan
Defined contribution plans
Percentage of salary matched, second level, percent		1.00%
Maximum \| IMG 401(k) Plan
Defined contribution plans
Percentage of salary matched, second level, percent		6.00%
UNITED KINGDOM
Defined contribution plans
Employer contribution as percentage of employees' salary		12.00%
Belgium \| Minimum
Defined contribution plans
Employer contribution as percentage of employees' salary		6.50%
Belgium \| Maximum
Defined contribution plans
Employer contribution as percentage of employees' salary		8.50%
Bermuda
Defined contribution plans
Employer contribution as percentage of employees' salary		10.00%
Employee vesting period		2 years
Bermuda \| Sirius Bermuda Insurance Company Ltd.
Defined contribution plans
Benefit plan expenses recognized		$ 0.6	$ 0.5	$ 0.4
Swisscanto plan \| Switzerland \| Minimum
Defined contribution plans
Employer share of total premium charges incurred, percent		60.00%
Employee share of total premium charges incurred, percent		30.00%
Swisscanto plan \| Switzerland \| Maximum
Defined contribution plans
Employer share of total premium charges incurred, percent		70.00%
Employee share of total premium charges incurred, percent		40.00%