Debt Obligations - Component (Details) - USD ($)		1 Months Ended	12 Months Ended
	Feb. 19, 2021	Jan. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Debt Obligations
Amount borrowed			$ 24,000,000	$ 207,900,000	$ 116,200,000
Applicable Interest Rate (as a percent)			3.96%
Outstanding Balance			$ 649,382,000	693,388,000
Available for borrowing			510,100,000	527,600,000
Payments on debt			$ 68,160,000	159,667,000	138,866,000
Minimum
Debt Obligations
Interest rate (as a percent)			5.00%
Maximum
Debt Obligations
Interest rate (as a percent)			7.50%
Bank Borrowings
Debt Obligations
Amount borrowed			$ 24,000,000	107,900,000	116,200,000
Applicable Interest Rate (as a percent)			1.38%
Outstanding Balance			$ 89,900,000	93,900,000
Available for borrowing			510,100,000	506,100,000
Payments on debt			$ 28,000,000	126,000,000	100,700,000
Bank Borrowings | Subsequent Event
Debt Obligations
Amount borrowed		$ 9,000,000
Outstanding Balance		98,900,000
Available for borrowing		$ 501,100,000
Senior Unsecured Notes
Debt Obligations
Amount borrowed				100,000,000
Applicable Interest Rate (as a percent)			4.37%
Outstanding Balance	$ 552,482,000		$ 559,482,000	599,488,000
Available for borrowing				21,500,000
Payments on debt	7,000,000		$ 40,160,000	$ 33,667,000	$ 38,166,000
Senior Unsecured Notes | Subsequent Event
Debt Obligations
Outstanding Balance	552,482,000
Payments on debt	$ 7,000,000
Senior Unsecured Notes | Minimum
Debt Obligations
Interest rate (as a percent)			3.85%
Senior Unsecured Notes | Maximum
Debt Obligations
Interest rate (as a percent)			5.03%