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Debt - Narrative (Details) £ in Millions							1 Months Ended				3 Months Ended		9 Months Ended
Debt - Narrative (Details) £ in Millions	Sep. 08, 2021 USD ($)	Jun. 02, 2021 USD ($)	Jun. 01, 2021	Apr. 07, 2020 USD ($)	Apr. 07, 2020 GBP (£)	Apr. 06, 2020 USD ($)	Sep. 30, 2021 USD ($) credit_facility	Aug. 31, 2021	Jul. 31, 2021	Mar. 31, 2007 USD ($)	Sep. 30, 2021 USD ($) credit_facility	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) credit_facility	Sep. 30, 2021 GBP (£)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 EUR (€) credit_facility	Aug. 11, 2021 USD ($)	Jul. 13, 2021 USD ($)	Jul. 13, 2021 EUR (€)	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 15, 2018 USD ($)	Mar. 31, 2007 EUR (€)
Debt Instrument
Debt							$ 710,400,000				$ 710,400,000		$ 710,400,000								$ 376,400,000
Debt covenant, maximum loan to value percent, period one							50.00%				50.00%		50.00%			50.00%
Debt covenant, maximum loan to value percent, period two							40.00%				40.00%		40.00%			40.00%
Debt covenant, maximum loan to value percent, period three							25.00%				25.00%		25.00%			25.00%
Debt covenant, maximum loan to value percent, period four							15.00%				15.00%		15.00%			15.00%
Debt covenant, maximum loan to value percent, period five							0.00%				0.00%		0.00%			0.00%
Debt covenant, maximum loan to value percent							34.00%				34.00%		34.00%			34.00%
Draw down of debt and revolving line of credit													$ 403,900,000		$ 69,400,000
Other
Debt Instrument
Secured credit facilities \| credit_facility							3				3		3			3
Specialty Insurance Distribution (NSM)
Debt Instrument
Debt							$ 295,000,000.0				$ 295,000,000.0		$ 295,000,000.0								272,600,000
Specialty Insurance Distribution (NSM) \| Kingsbridge Group Limited
Debt Instrument
Borrowings				$ 52,400,000	£ 42.5
Unsecured Subordinated Debt \| Group Ark Insurance Limited
Debt Instrument
Borrowings											157,800,000
Unsecured Subordinated Debt \| Group Ark Insurance Limited \| Ark
Debt Instrument
Notes issued							163,300,000				163,300,000		163,300,000
Ark 2007 Notes Tranche 1 \| Ark Subordinated Debt
Debt Instrument
Outstanding balance							30,000,000.0				30,000,000.0		30,000,000.0
Ark 2007 Notes Tranche 1 \| Junior Notes \| Ark
Debt Instrument
Amount borrowed										$ 30,000,000
Debt							30,000,000				30,000,000		30,000,000
Ark 2007 Notes Tranche 1 \| Junior Notes \| USD-LIBOR \| Ark
Debt Instrument
Basis spread on variable rate										4.60%
Ark 2007 Notes Tranche 2 \| Ark Subordinated Debt
Debt Instrument
Outstanding balance							13,900,000				13,900,000		13,900,000
Ark 2007 Notes Tranche 2 \| Junior Notes \| Ark
Debt Instrument
Amount borrowed \| €																									€ 12,000,000
Debt							13,900,000				13,900,000		13,900,000			€ 12,000,000
Ark 2007 Notes Tranche 2 \| Junior Notes \| EURIBOR \| Ark
Debt Instrument
Basis spread on variable rate										4.60%
Ark 2021 Notes Tranche 1
Debt Instrument
Outstanding balance \| €																€ 39,100,000
Ark 2021 Notes Tranche 1 \| Ark Subordinated Debt
Debt Instrument
Outstanding balance							45,300,000				45,300,000		45,300,000
Ark 2021 Notes Tranche 1 \| Unsecured Subordinated Debt \| Group Ark Insurance Limited
Debt Instrument
Notes issued																		$ 46,300,000	€ 39,100,000
Ark 2021 Notes Tranche 1 \| Unsecured Subordinated Debt \| Group Ark Insurance Limited \| EURIBOR
Debt Instrument
Basis spread on variable rate									5.75%
Ark 2021 Notes Tranche 2 \| Ark Subordinated Debt
Debt Instrument
Outstanding balance							47,000,000.0				47,000,000.0		47,000,000.0
Ark 2021 Notes Tranche 2 \| Unsecured Subordinated Debt \| Group Ark Insurance Limited
Debt Instrument
Notes issued																	$ 47,000,000
Ark 2021 Notes Tranche 2 \| Unsecured Subordinated Debt \| Group Ark Insurance Limited \| USD-LIBOR
Debt Instrument
Basis spread on variable rate								5.75%
Ark 2021 Notes Tranche 3 \| Ark Subordinated Debt
Debt Instrument
Outstanding balance							$ 70,000,000				70,000,000		70,000,000
Ark 2021 Notes Tranche 3 \| Unsecured Subordinated Debt \| Group Ark Insurance Limited
Debt Instrument
Notes issued	$ 70,000,000
Additional interest rate	1.00%
Solvency capital requirement							120.00%
Debt to capital requirement							40.00%
Ark 2021 Notes Tranche 3 \| Unsecured Subordinated Debt \| Group Ark Insurance Limited \| USD-LIBOR
Debt Instrument
Basis spread on variable rate	6.10%
Ark \| Ark Subordinated Debt
Debt Instrument
Debt							$ 200,700,000				200,700,000		200,700,000
Ark \| Term Loan
Debt Instrument
Line of credit							45,000,000				45,000,000		45,000,000
Total commitment under revolving credit facility							150,000,000				150,000,000		150,000,000
NSM Bank Facility \| Specialty Insurance Distribution (NSM)
Debt Instrument
Debt							300,400,000				300,400,000	$ 275,000,000.0	$ 300,400,000		275,000,000.0					$ 276,600,000	277,400,000	$ 273,400,000	$ 221,300,000
Total commitment under revolving credit facility				291,400,000		$ 234,000,000
Percentage of discounted future cash flows													10.00%	10.00%
NSM Bank Facility \| Term Loan
Debt Instrument
Debt							293,700,000				293,700,000		$ 293,700,000								271,300,000
Outstanding balance							300,400,000				300,400,000		300,400,000								277,400,000
NSM Bank Facility \| Term Loan \| Specialty Insurance Distribution (NSM)
Debt Instrument
Amount borrowed		$ 50,000,000		276,400,000		$ 224,000,000	274,400,000				274,400,000		274,400,000
Borrowings				$ 52,400,000	£ 42.5						0	0	0		52,400,000
Floor rate				1.25%	1.25%
NSM Bank Facility \| Term Loan \| Specialty Insurance Distribution (NSM) \| Kingsbridge Group Limited
Debt Instrument
Borrowings												52,400,000			52,400,000
NSM Bank Facility \| Term Loan \| Specialty Insurance Distribution (NSM) \| Interest Rate Swap
Debt Instrument
Total commitment under revolving credit facility																								$ 151,000,000.0
Hedged liability, fair value																								$ 151,000,000
NSM Bank Facility \| Term Loan \| Specialty Insurance Distribution (NSM) \| Interest Rate Swap \| Term loan - hedged
Debt Instrument
Debt							146,500,000				146,500,000		146,500,000
NSM Bank Facility \| Term Loan \| Specialty Insurance Distribution (NSM) \| Interest Rate Swap \| Term loan - unhedged
Debt Instrument
Debt							128,000,000				128,000,000		128,000,000
NSM Bank Facility \| Term Loan \| Specialty Insurance Distribution (NSM) \| USD-LIBOR
Debt Instrument
Floor rate				1.25%	1.25%
NSM Bank Facility \| Term Loan \| Specialty Insurance Distribution (NSM) \| USD-LIBOR \| Minimum
Debt Instrument
Basis spread on variable rate		4.50%	5.50%	5.50%	5.50%	4.25%
NSM Bank Facility \| Term Loan \| Specialty Insurance Distribution (NSM) \| USD-LIBOR \| Maximum
Debt Instrument
Basis spread on variable rate		5.00%	6.00%	6.00%	6.00%	4.75%
NSM Bank Facility \| Term Loan \| Specialty Insurance Distribution (NSM) \| GBP-LIBOR \| Minimum
Debt Instrument
Basis spread on variable rate		5.00%	6.00%
NSM Bank Facility \| Term Loan \| Specialty Insurance Distribution (NSM) \| GBP-LIBOR \| Maximum
Debt Instrument
Basis spread on variable rate		5.50%	6.50%
NSM Bank Facility \| GBP Term Loan \| Specialty Insurance Distribution (NSM)
Debt Instrument
Borrowings													56,600,000	£ 41.9
NSM Bank Facility \| Revolving credit loan \| Specialty Insurance Distribution (NSM)
Debt Instrument
Amount borrowed							26,000,000				26,000,000		26,000,000
Borrowings											35,000,000.0	$ 0	35,000,000.0		$ 0
Total commitment under revolving credit facility		$ 40,000,000		$ 15,000,000		$ 10,000,000
Other NSM
Debt Instrument
Debt							1,300,000				1,300,000		1,300,000								1,300,000
Other NSM \| Term Loan \| Specialty Insurance Distribution (NSM)
Debt Instrument
Debt							1,400,000				1,400,000		1,400,000
Kudu Bank Facility \| Kudu
Debt Instrument
Total commitment under revolving credit facility							300,000,000				300,000,000		300,000,000
Draw down of debt and revolving line of credit											101,000,000		203,000,000
Kudu Bank Facility \| Term Loan
Debt Instrument
Debt																					86,300,000
Outstanding balance																					$ 89,200,000
Kudu Bank Facility \| Term Loan \| Kudu
Debt Instrument
Amount borrowed							$ 97,000,000				$ 97,000,000		$ 97,000,000
Percentage of borrowing base							35.00%				35.00%		35.00%			35.00%
Remaining borrowing capacity under revolving credit facility							$ 11,700,000				$ 11,700,000		$ 11,700,000
Kudu Bank Facility \| Revolving credit loan \| Kudu
Debt Instrument
Loss on debt extinguishment													4,100,000
Draw down of debt and revolving line of credit													3,000,000
Repayments on credit facility													92,200,000
Kudu Bank Facility \| Line of Credit \| Kudu
Debt Instrument
Interest reserve							4,500,000				4,500,000		$ 4,500,000
Kudu Bank Facility \| Line of Credit \| Kudu \| USD-LIBOR
Debt Instrument
Basis spread on variable rate													4.30%	4.30%
Kudu Bank Facility \| Line of Credit \| Kudu \| Base Rate
Debt Instrument
Basis spread on variable rate													0.25%	0.25%
First Credit Facility \| Other
Debt Instrument
Total commitment under revolving credit facility							16,300,000				16,300,000		$ 16,300,000
First Credit Facility \| Term Loan \| Other
Debt Instrument
Total commitment under revolving credit facility							11,300,000				11,300,000		11,300,000
First Credit Facility \| Revolving credit loan \| Other
Debt Instrument
Total commitment under revolving credit facility							2,000,000				2,000,000		2,000,000
First Credit Facility \| Delayed Draw Term Loans \| Other
Debt Instrument
Total commitment under revolving credit facility							3,000,000				3,000,000		3,000,000
Second Credit Facility \| Other
Debt Instrument
Total commitment under revolving credit facility							15,000,000				15,000,000		15,000,000
Second Credit Facility \| Term Loan \| Other
Debt Instrument
Total commitment under revolving credit facility							9,000,000				9,000,000		9,000,000
Second Credit Facility \| Revolving credit loan \| Other
Debt Instrument
Total commitment under revolving credit facility							2,000,000				2,000,000		2,000,000
Second Credit Facility \| Delayed Draw Term Loans \| Other
Debt Instrument
Total commitment under revolving credit facility							4,000,000				4,000,000		4,000,000
Third Credit Facility \| Other
Debt Instrument
Total commitment under revolving credit facility							4,000,000				4,000,000		4,000,000
Third Credit Facility \| Term Loan \| Other
Debt Instrument
Outstanding balance							$ 19,500,000				19,500,000		19,500,000
Repayments of lines of credit											1,200,000		2,600,000
Third Credit Facility \| Revolving credit loan \| Other
Debt Instrument
Draw down of debt and revolving line of credit											$ 400,000		$ 700,000