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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Debt - Narrative (Details) - USD ($)				3 Months Ended			6 Months Ended
Debt - Narrative (Details) - USD ($)	Apr. 07, 2020	Apr. 01, 2019	Dec. 03, 2018	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Dec. 23, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 15, 2018	May 11, 2018
Debt Instrument
Fair value investments				$ 2,656,900,000		$ 2,483,000,000.0	$ 2,656,900,000	$ 2,483,000,000.0	$ 2,746,000,000.0			$ 2,328,700,000
Total debt				349,800,000			349,800,000		283,500,000
Total commitment under revolving credit facility	$ 291.4				$ 234,000,000.0
Repayments of debt							(1,100,000)	(21,700,000)
NSM Bank Facility
Debt Instrument
Face amount														$ 100,000,000.0
Other Operations
Debt Instrument
Total commitment under revolving credit facility				16,300,000			16,300,000
NSM Holding Co LLC
Debt Instrument
Total debt				268,400,000			268,400,000		219,200,000
NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Total debt				267,000,000.0			267,000,000.0		217,400,000
Face amount				$ 273,400,000	220,700,000	222,300,000	$ 273,400,000	222,300,000	$ 221,300,000		$ 186,400,000	$ 180,400,000
Percentage of discounted future cash flows							10.00%
Debt, weighted average interest rate, all-in				7.21%			7.21%		6.60%
Kudu \| Kudu Bank Facility [Member]
Debt Instrument
Total commitment under revolving credit facility										$ 124,000,000.0
Kudu \| Kudu Debt [Member]
Debt Instrument
Total debt				$ 70,800,000			$ 70,800,000		$ 53,600,000
Term Loan \| Other Operations
Debt Instrument
Total commitment under revolving credit facility				11,300,000			11,300,000
Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Face amount	276,400,000			$ 273,400,000	224,000,000.0		$ 273,400,000		$ 221,300,000
Effective percentage				7.20%			7.20%		7.50%
Unamortized issuance cost				$ (6,400,000)			$ (6,400,000)		$ (3,900,000)
Repayments of debt				0		(500,000)	(600,000)	(1,000,000.0)
Proceeds from borrowings				52,400,000		42,900,000	52,400,000	42,900,000
Debt Instrument, Increase (Decrease), Other, Net				300,000		0	300,000	0
Term Loan \| NSM Holding Co LLC \| Other NSM
Debt Instrument
Total debt				1,400,000			1,400,000		$ 1,800,000
Face amount				$ 1,600,000			$ 1,600,000
Effective percentage				3.20%			3.20%		3.00%
Term Loan \| Kudu \| Kudu Bank Facility [Member]
Debt Instrument
Face amount				$ 74,000,000.0			$ 74,000,000.0		$ 57,000,000.0
Total commitment under revolving credit facility										57,000,000.0
Proceeds from Issuance of Long-term Debt				$ 5,000,000.0			$ 17,000,000.0
Term Loan \| Kudu \| Kudu Debt [Member]
Debt Instrument
Effective percentage				8.10%			8.10%		8.30%
Unamortized issuance cost				$ (3,200,000)			$ (3,200,000)		$ (3,400,000)
Term Loan \| Other \| Other
Debt Instrument
Total debt				10,600,000			10,600,000		10,700,000
Face amount				$ 10,900,000			$ 10,900,000		$ 11,100,000
Effective percentage				9.30%			9.30%		8.30%
Unamortized issuance cost				$ (300,000)			$ (300,000)		$ (400,000)
Delayed Draw Term Loans [Member] \| Other Operations
Debt Instrument
Total commitment under revolving credit facility				3,000,000.0			3,000,000.0
Delayed Draw Term Loans [Member] \| Kudu \| Kudu Bank Facility [Member]
Debt Instrument
Total commitment under revolving credit facility										62,000,000.0
Delayed Draw Term Loans [Member] \| Kudu \| Kudu Debt [Member]
Debt Instrument
Total commitment under revolving credit facility									125,000,000.0
Revolving Credit Facility \| Other Operations
Debt Instrument
Total commitment under revolving credit facility				2,000,000.0			2,000,000.0
Revolving Credit Facility \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Total commitment under revolving credit facility	15,000,000.0				$ 10,000,000.0
Repayments of debt				0		(6,500,000)	0	(6,500,000)
Proceeds from borrowings				$ 0		0	$ 0	6,500,000
Revolving Credit Facility \| Kudu \| Kudu Bank Facility [Member]
Debt Instrument
Total commitment under revolving credit facility										$ 5,000,000.0
Interest Rate Swap \| NSM Holding Co LLC
Debt Instrument
Derivative, fixed interest rate													2.97%
Derivative, variable interest rate				0.17%			0.17%
Interest Rate Swap \| Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Total debt				$ 148,400,000			$ 148,400,000
Total commitment under revolving credit facility													$ 151,000,000.0
Hedged Liability, Fair Value Hedge													$ 151,000,000.0
Derivative, fixed interest rate					2.97%
Debt, weighted average interest rate, all-in				6.78%			6.78%
London Interbank Offered Rate (LIBOR) \| Line of Credit [Member] \| Kudu \| Kudu Bank Facility [Member]
Debt Instrument
Basis spread on variable rate							1.00%
London Interbank Offered Rate (LIBOR) \| Minimum \| Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Debt Instrument, Interest Rate, Increase (Decrease)							1.00%
Basis spread on variable rate					4.25%		5.50%
London Interbank Offered Rate (LIBOR) \| Minimum \| Line of Credit [Member] \| Kudu \| Kudu Bank Facility [Member]
Debt Instrument
Basis spread on variable rate							5.50%
London Interbank Offered Rate (LIBOR) \| Maximum \| Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Debt Instrument, Interest Rate, Increase (Decrease)							1.25%
Basis spread on variable rate					4.75%		6.00%
London Interbank Offered Rate (LIBOR) \| Maximum \| Line of Credit [Member] \| Kudu \| Kudu Bank Facility [Member]
Debt Instrument
Basis spread on variable rate							6.25%
Prime Rate \| Line of Credit [Member] \| Kudu \| Kudu Bank Facility [Member]
Debt Instrument
Basis spread on variable rate							1.00%
Prime Rate \| Minimum \| Line of Credit [Member] \| Kudu \| Kudu Bank Facility [Member]
Debt Instrument
Basis spread on variable rate							4.50%
Prime Rate \| Maximum \| Line of Credit [Member] \| Kudu \| Kudu Bank Facility [Member]
Debt Instrument
Basis spread on variable rate							5.25%
Designated as Hedging Instrument \| Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Debt, weighted average interest rate, all-in				6.67%			6.67%
Designated as Hedging Instrument \| Interest Rate Swap \| Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Total debt				$ 148,400,000			$ 148,400,000
Debt, weighted average interest rate, all-in				8.72%			8.72%
Not Designated as Hedging Instrument \| Interest Rate Swap \| Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Total debt				$ 125,000,000.0			$ 125,000,000.0
Effective percentage				6.90%			6.90%
Kingsbridge [Member] \| Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Proceeds from borrowings				$ 52,400,000
Embrace \| NSM Holding Co LLC
Debt Instrument
Proceeds from borrowings		$ 20,400,000
Embrace \| Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Proceeds from borrowings						$ 20,400,000
KBK Insurance Group, Inc. & KBK Premium Services, Inc.
Debt Instrument
Proceeds from borrowings			$ 30,100,000
Renewal Rights \| Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Proceeds from borrowings								$ 22,500,000
Carrying Value \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Total debt				267,000,000.0			$ 267,000,000.0		217,400,000
Debt securities issued by corporations
Debt Instrument
Fair value investments				505,800,000			505,800,000		467,200,000
Debt securities issued by corporations \| AAA
Debt Instrument
Fair value investments				8,300,000			8,300,000		9,500,000
Debt securities issued by corporations \| AA
Debt Instrument
Fair value investments				53,600,000			53,600,000		73,900,000
Debt securities issued by corporations \| Standard & Poor's, A Rating [Member]
Debt Instrument
Fair value investments				314,100,000			314,100,000		288,500,000
Debt securities issued by corporations \| BBB
Debt Instrument
Fair value investments				128,100,000			128,100,000		95,300,000
Debt securities issued by corporations \| BB
Debt Instrument
Fair value investments				$ 1,700,000			$ 1,700,000		$ 0
United Kingdom, Pounds \| Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Face amount	42,500,000
United Kingdom, Pounds \| London Interbank Offered Rate (LIBOR) \| Minimum \| Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Basis spread on variable rate							6.00%
United Kingdom, Pounds \| London Interbank Offered Rate (LIBOR) \| Maximum \| Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Debt Instrument, Interest Rate, Increase (Decrease)							1.25%
Basis spread on variable rate							6.50%
Derivative, Cap Interest Rate				1.25%			1.25%
United Kingdom, Pounds \| Kingsbridge [Member] \| NSM Holding Co LLC
Debt Instrument
Proceeds from borrowings	42,500,000
United States of America, Dollars \| Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Face amount	52,400,000
United States of America, Dollars \| Kingsbridge [Member] \| NSM Holding Co LLC
Debt Instrument
Proceeds from borrowings	$ 52,400,000