Fair Value Measurements - Schedule of Differences between Fair Value and Principal Balance for Loans Held for Sale Measured at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Mortgage loans held for sale measured at fair value	$ 374,916	$ 482,089
Past due loans of 90 days or more		320
Nonaccrual loans		741
Aggregate Unpaid Principal Balance
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Mortgage loans held for sale measured at fair value	364,063	467,039
Past due loans of 90 days or more		320
Nonaccrual loans		741
Difference
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Mortgage loans held for sale measured at fair value	$ 10,853	$ 15,050