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Derivatives - Interest Rate Swap (Details) - USD ($)	8 Months Ended	12 Months Ended
Derivatives - Interest Rate Swap (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2018	Jun. 15, 2018	May 11, 2018	Dec. 31, 2017
Derivative [Line Items]
Debt	$ 192,700,000	$ 192,700,000			$ 23,800,000
NSM Holding Co LLC
Derivative [Line Items]
Debt	$ 178,500,000	$ 178,500,000			0
NSM Bank Facility \| NSM Holding Co LLC
Derivative [Line Items]
Total commitment under revolving credit facility				$ 100,000,000
NSM Bank Facility \| NSM Holding Co LLC \| Term Loan
Derivative [Line Items]
Debt					$ 0
NSM Bank Facility \| NSM Holding Co LLC \| Term Loan \| London Interbank Offered Rate (LIBOR) \| Minimum
Derivative [Line Items]
Basis spread on variable rate	4.25%	4.25%
NSM Bank Facility \| NSM Holding Co LLC \| Term Loan \| London Interbank Offered Rate (LIBOR) \| Maximum
Derivative [Line Items]
Basis spread on variable rate	4.75%	4.75%
NSM Bank Facility \| NSM Holding Co LLC \| Term Loan \| Prime Rate \| Minimum
Derivative [Line Items]
Basis spread on variable rate	3.25%	3.25%
NSM Bank Facility \| NSM Holding Co LLC \| Term Loan \| Prime Rate \| Maximum
Derivative [Line Items]
Basis spread on variable rate	3.75%	3.75%
Interest Rate Swap \| NSM Holding Co LLC
Derivative [Line Items]
Fixed interest rate	2.97%	2.97%
Variable interest rate	2.52%	2.52%
Net interest expense	$ 700,000.0
Derivative, fair value, net	2,700,000.0	$ 2,700,000.0
Change in the fair value of the swap	$ (2,700,000.0)
Interest Rate Swap \| NSM Bank Facility \| NSM Holding Co LLC \| Term Loan
Derivative [Line Items]
Total commitment under revolving credit facility			$ 151,000,000
Fixed interest rate	2.97%	2.97%
Variable interest rate	2.52%	2.52%
Debt	$ 150,200,000	$ 150,200,000
Debt, weighted average interest rate	7.42%	7.42%
Designated as Hedging Instrument \| Interest Rate Swap \| NSM Bank Facility \| NSM Holding Co LLC \| Term Loan
Derivative [Line Items]
Debt	$ 150,200,000	$ 150,200,000
Debt, weighted average interest rate	7.47%	7.47%