Derivatives - NSM Interest Rate Swap and Cap (Details)		3 Months Ended		9 Months Ended
	Aug. 18, 2020	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 04, 2020 GBP (£)	Jun. 04, 2020 USD ($)	Apr. 07, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jun. 15, 2018 USD ($)	May 11, 2018 USD ($)
Derivative [Line Items]
Total commitment under revolving credit facility									$ 291,400,000	$ 234,000,000.0
Debt		$ 359,400,000		$ 359,400,000							$ 283,500,000
NSM Bank Facility
Derivative [Line Items]
Amount borrowed															$ 100,000,000.0
NSM Bank Facility | Term Loan
Derivative [Line Items]
Debt		268,800,000		268,800,000							217,400,000
Specialty Insurance Distribution (NSM)
Derivative [Line Items]
Debt		$ 270,200,000		$ 270,200,000							219,200,000
Specialty Insurance Distribution (NSM) | Interest Rate Swap
Derivative [Line Items]
Derivative, fixed interest rate														2.97%
Derivative, variable interest rate		1.00%		1.00%
Interest expense, net		$ 600,000	$ 300,000	$ 1,900,000	$ 600,000
Derivative, fair value, net			8,900,000		8,900,000						6,600,000
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax		500,000	(800,000)	(2,300,000)	(5,100,000)
Specialty Insurance Distribution (NSM) | Interest Rate Cap
Derivative [Line Items]
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments				(100,000)
Derivative, fair value, net		$ 100,000		100,000
Derivative, Cost of Hedge				$ 100,000
GBP-LIBOR rate		0.06%		0.06%
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax		$ (100,000)
Specialty Insurance Distribution (NSM) | Interest Rate Cap | GBP-LIBOR
Derivative [Line Items]
Derivative, cap interest rate		1.25%		1.25%
Specialty Insurance Distribution (NSM) | NSM Bank Facility
Derivative [Line Items]
Debt		$ 275,000,000.0	$ 221,900,000	$ 275,000,000.0	$ 221,900,000	$ 273,400,000					$ 221,300,000	$ 222,300,000	$ 180,400,000
Debt, weighted average interest rate, all-in		6.91%		6.91%							6.60%
Amount borrowed		$ 275,000,000.0		$ 275,000,000.0
Specialty Insurance Distribution (NSM) | NSM Bank Facility | Term Loan
Derivative [Line Items]
Amount borrowed							£ 42,500,000	$ 52,400,000	$ 276,400,000	$ 224,000,000.0
Specialty Insurance Distribution (NSM) | NSM Bank Facility | Term Loan | Designated as Hedging Instrument
Derivative [Line Items]
Debt, weighted average interest rate, all-in		6.84%		6.84%
Specialty Insurance Distribution (NSM) | NSM Bank Facility | Interest Rate Swap | Term Loan
Derivative [Line Items]
Total commitment under revolving credit facility														$ 151,000,000.0
Derivative, fixed interest rate														2.97%
Debt		$ 148,000,000.0		$ 148,000,000.0
Debt, weighted average interest rate, all-in						6.98%
Specialty Insurance Distribution (NSM) | NSM Bank Facility | Interest Rate Swap | Term Loan | Designated as Hedging Instrument
Derivative [Line Items]
Debt		$ 148,000,000.0		$ 148,000,000.0
Debt, weighted average interest rate, all-in		8.72%		8.72%
Specialty Insurance Distribution (NSM) | NSM Bank Facility | Interest Rate Cap
Derivative [Line Items]
Amount borrowed							£ 42,500,000	$ 52,400,000
Debt instrument, term extension	1 year