Fair Value - Fair Value Option Maturities, Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018
Fair value
Total fair value	$ 4,912,953	$ 4,522,971	$ 3,506,406	$ 2,668,929	$ 2,521,647	$ 2,416,955	$ 2,527,231	$ 2,584,236
Recurring basis
Fair value
Total fair value	4,912,953				2,521,647
Loans held for sale
Fair value
Current through 89 days delinquent	4,628,333				2,324,203
Not in foreclosure	236,650				143,631
In foreclosure	47,970				53,813
Total fair value	4,912,953				2,521,647
Principal amount due upon maturity
Current through 89 days delinquent	4,431,854				2,220,371
Not in foreclosure	241,958				144,011
In foreclosure	50,194				56,254
Total principal amount due upon maturity	4,724,006				2,420,636
Difference
Current through 89 days delinquent	196,479				103,832
Not in foreclosure	(5,308)				(380)
In foreclosure	(2,224)				(2,441)
Total difference	$ 188,947				$ 101,011