Fair Value - Measurement Basis, Nonrecurring (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Total gains (losses) on assets measured at estimated fair values on a nonrecurring basis
Notes Payable	$ 1,296,285		$ 1,295,840
Term Notes
Total gains (losses) on assets measured at estimated fair values on a nonrecurring basis
Notes Payable	1,296,285		1,295,840
Unsecured senior note
Total gains (losses) on assets measured at estimated fair values on a nonrecurring basis
Notes Payable	1,288,198		645,820
Nonrecurring basis
Financial statement items measured at fair value on a nonrecurring basis
Real estate acquired in settlement of loans	2,568		1,450
Total gains (losses) on assets measured at estimated fair values on a nonrecurring basis
Real estate acquired in settlement of loans	(412)	$ (3,980)
Nonrecurring basis | Level 3
Financial statement items measured at fair value on a nonrecurring basis
Real estate acquired in settlement of loans	2,568		1,450
Fair Values | Term Notes
Total gains (losses) on assets measured at estimated fair values on a nonrecurring basis
Notes Payable	1,296,344		1,268,304
Fair Values | Unsecured senior note
Total gains (losses) on assets measured at estimated fair values on a nonrecurring basis
Notes Payable	$ 1,290,250		$ 685,750