Fair Value - Measurement Basis, Nonrecurring (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Total gains (losses) on assets measured at estimated fair values on a nonrecurring basis
Notes Payable	$ 1,295,143		$ 1,295,143		$ 1,294,070
Term Notes
Total gains (losses) on assets measured at estimated fair values on a nonrecurring basis
Notes Payable	1,295,143		1,295,143		1,294,070
Unsecured senior note
Total gains (losses) on assets measured at estimated fair values on a nonrecurring basis
Notes Payable	492,358		492,358
Nonrecurring basis
Financial statement items measured at fair value on a nonrecurring basis
Real estate acquired in settlement of loans	7,346		7,346		9,850
Total gains (losses) on assets measured at estimated fair values on a nonrecurring basis
Real estate acquired in settlement of loans	(825)	$ 139	(2,059)	$ 162
Nonrecurring basis | Level 3
Financial statement items measured at fair value on a nonrecurring basis
Real estate acquired in settlement of loans	7,346		7,346		9,850
Fair Values | Term Notes
Total gains (losses) on assets measured at estimated fair values on a nonrecurring basis
Notes Payable	1,259,619		1,259,619		$ 1,303,047
Fair Values | Unsecured senior note
Total gains (losses) on assets measured at estimated fair values on a nonrecurring basis
Notes Payable	$ 510,000		$ 510,000