Fair Value Measurements - Summarizes Differences Between Fair Value and Principal Balance for Mortgage Loans Held for Sale Measure at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Summarizes differences between fair value and principal balance for mortgage loans held for sale measure at fair value
Mortgage loans held for sale measured at fair value	$ 252,025	$ 108,316
Aggregate Fair Value
Summarizes differences between fair value and principal balance for mortgage loans held for sale measure at fair value
Mortgage loans held for sale measured at fair value	252,025
Past due loans of 90 days or more	0
Nonaccrual loans	0
Aggregate Unpaid Principal Balance
Summarizes differences between fair value and principal balance for mortgage loans held for sale measure at fair value
Mortgage loans held for sale measured at fair value	246,806
Past due loans of 90 days or more	0
Nonaccrual loans	0
Difference
Summarizes differences between fair value and principal balance for mortgage loans held for sale measure at fair value
Mortgage loans held for sale measured at fair value	5,219
Past due loans of 90 days or more	0
Nonaccrual loans	$ 0