Fair Value - Fair Value Option Maturities, Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017
Fair value
Total fair value	$ 2,521,647	$ 2,416,955	$ 2,527,231	$ 2,584,236	$ 3,099,103	$ 2,935,593	$ 3,037,602	$ 2,277,751
Recurring basis
Fair value
Total fair value	2,521,647				3,099,103
Mortgage loans held for sale
Fair value
Current through 89 days delinquent	2,324,203				2,430,517
Not in foreclosure	143,631				614,329
In foreclosure	53,813				54,257
Total fair value	2,521,647				3,099,103
Principal amount due upon maturity
Current through 89 days delinquent	2,220,371				2,326,772
Not in foreclosure	144,011				614,357
In foreclosure	56,254				57,248
Total principal amount due upon maturity	2,420,636				2,998,377
Difference
Current through 89 days delinquent	103,832				103,745
Not in foreclosure	(380)				(28)
In foreclosure	(2,441)				(2,991)
Total difference	$ 101,011				$ 100,726