Fair Value - Measurement Basis, Nonrecurring (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Total gains (losses) on assets measured at estimated fair values on a nonrecurring basis
Notes Payable	$ 1,292,291	$ 891,505
Nonrecurring basis
Financial statement items measured at fair value on a nonrecurring basis
Mortgage servicing rights at lower of amortized cost or fair value		1,463,552
Real estate acquired in settlement of loans	2,150	2,355
Total assets		1,465,907
Total gains (losses) on assets measured at estimated fair values on a nonrecurring basis
Mortgage servicing rights at lower of amortized cost or fair value		(6,853)	$ (60,487)
Real estate acquired in settlement of loans	(75)	(125)	(86)
Total gains on assets measured at estimated fair values on a nonrecurring basis	(75)	(6,978)	$ (60,487)
Nonrecurring basis | Level 3
Financial statement items measured at fair value on a nonrecurring basis
Mortgage servicing rights at lower of amortized cost or fair value		1,463,552
Real estate acquired in settlement of loans	2,150	2,355
Total assets		1,465,907
Fair Values
Total gains (losses) on assets measured at estimated fair values on a nonrecurring basis
Notes Payable	$ 1,300,000	$ 903,900