Stockholder's Equity - Initial public offering (Details) - USD ($) $ in Thousands		12 Months Ended
	Oct. 01, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019
Class of Stock [Line Items]
Proceeds from Public Offering of Class A Shares, net of underwriting discounts and offering costs		$ 73,914	$ 104,010
Repayments of Notes Payable			$ 55,960
Common stock, shares outstanding		21,620,000		23,554,000
Class A common stock
Class of Stock [Line Items]
Common Stock, Shares, Issued	5,750,000	21,620,000		23,554,000
Common stock, shares outstanding		21,620,000		23,554,000
Class A common stock | IPO
Class of Stock [Line Items]
Number of shares issued	2,000,000
Proceeds from issuance of shares after deducting underwriting discounts and commissions and offering expenses	$ 73,900
Class A common stock | Over allotment
Class of Stock [Line Items]
Number of shares issued	3,750
Class B common stock
Class of Stock [Line Items]
Common Stock, Shares, Issued		6,547,000		4,752,000
Common stock, shares outstanding		6,547,000		4,752,000
Amended Holdco LLC Agreement
Class of Stock [Line Items]
Common stock, shares outstanding		28,167,000		28,306,000