Fair Value Measurements - Summary of Differences Between Fair Value and Principal Value (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage loans held for sale	$ 219,848	$ 108,316
Aggregate Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage loans held for sale	219,848
Past due loans of 90 days or more	0
Nonaccrual loans	0
Aggregate Unpaid Principal Balance
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage loans held for sale	211,460
Past due loans of 90 days or more	0
Nonaccrual loans	0
Difference
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage loans held for sale	8,388
Past due loans of 90 days or more	0
Nonaccrual loans	$ 0