Subsequent Events - Additional Information (Details)				3 Months Ended
	May 03, 2019 USD ($)	Apr. 30, 2019 USD ($) ft²	Feb. 19, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Subsequent Event [Line Items]
Option to extend lease term				true
Lease renewal term, operating lease				7 years
Letter of credit aggregate amount				$ 26,400,000	$ 26,400,000
Morgan Stanley Senior Funding, Inc | Credit Agreement | Term Loan A Facility
Subsequent Event [Line Items]
Debt instrument term, years			4 years
Maximum borrowing capacity			$ 75,000,000
Morgan Stanley Senior Funding, Inc | Credit Agreement | Revolving Credit Facility
Subsequent Event [Line Items]
Debt instrument term, years			4 years
Maximum borrowing capacity			$ 75,000,000
Morgan Stanley Senior Funding, Inc | Credit Agreement | Revolving Credit Facility | Term Loan A Facility
Subsequent Event [Line Items]
Maximum borrowing capacity			$ 75,000,000
Subsequent Event
Subsequent Event [Line Items]
Lease agreement date		Apr. 30, 2019
Lease commencement date		Jan. 01, 2021
Square feet of office space leased in five story office building | ft²		162,557
Remainder of lease term		Dec. 31, 2029
Option to extend lease term		true
Lease renewal term, operating lease		5 years
Letter of credit aggregate amount		$ 4,100,000
Subsequent Event | Morgan Stanley Senior Funding, Inc | Credit Agreement | Term Loan A Facility
Subsequent Event [Line Items]
Debt instrument face amount	$ 25,000,000
Debt instrument term, years	4 years
Maximum borrowing capacity	$ 100,000,000
Subsequent Event | Morgan Stanley Senior Funding, Inc | Credit Agreement | Revolving Credit Facility
Subsequent Event [Line Items]
Debt instrument face amount	$ 25,000,000
Debt instrument term, years	4 years
Maximum borrowing capacity	$ 100,000,000