Fair Value - Measurement Basis, Nonrecurring (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Total gains (losses) on assets measured at estimated fair values on a nonrecurring basis
Notes Payable	$ 1,296,731		$ 1,296,731		$ 1,295,840
Term Notes
Total gains (losses) on assets measured at estimated fair values on a nonrecurring basis
Notes Payable	1,296,731		1,296,731		1,295,840
Unsecured senior note
Total gains (losses) on assets measured at estimated fair values on a nonrecurring basis
Notes Payable	1,288,769		1,288,769		645,820
Nonrecurring basis
Financial statement items measured at fair value on a nonrecurring basis
Real estate acquired in settlement of loans	2,863		2,863		1,450
Total gains (losses) on assets measured at estimated fair values on a nonrecurring basis
Real estate acquired in settlement of loans	(434)	$ (1,001)	(743)	$ (2,283)
Nonrecurring basis | Level 3
Financial statement items measured at fair value on a nonrecurring basis
Real estate acquired in settlement of loans	2,863		2,863		1,450
Fair Values | Term Notes
Total gains (losses) on assets measured at estimated fair values on a nonrecurring basis
Notes Payable	1,298,603		1,298,603		1,268,304
Fair Values | Unsecured senior note
Total gains (losses) on assets measured at estimated fair values on a nonrecurring basis
Notes Payable	$ 1,309,750		$ 1,309,750		$ 685,750