Subsequent Events - Debt Obligations (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
Payments on debt			68,160,000	159,667,000	138,866,000
Outstanding Balance			649,382,000	693,388,000
Available for borrowing			510,100,000	527,600,000
Senior Unsecured Notes
Debt Obligations
Amount borrowed				100,000,000
Payments on debt	$ 7,000,000		40,160,000	33,667,000	38,166,000
Outstanding Balance	552,482,000		559,482,000	599,488,000
Available for borrowing				21,500,000
Bank Borrowings
Debt Obligations
Amount borrowed			24,000,000	107,900,000	116,200,000
Payments on debt			28,000,000	126,000,000	$ 100,700,000
Outstanding Balance			89,900,000	93,900,000
Available for borrowing			$ 510,100,000	$ 506,100,000
Subsequent Event | Senior Unsecured Notes
Debt Obligations
Payments on debt	7,000,000
Outstanding Balance	$ 552,482,000
Subsequent Event | Bank Borrowings
Debt Obligations
Amount borrowed		$ 9,000,000
Outstanding Balance		98,900,000
Available for borrowing		$ 501,100,000