Fair Value Measurements - Assets and Liabilities Measured on Non-Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale, at fair value	$ 8,081	$ 0
Real estate owned	92	959
Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale, at fair value	0	0
Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale, at fair value	0	0
Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale, at fair value	4,841,830	4,367,945
Measured on non-recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale, at fair value	1,525	10,251
Real estate owned	92	959
Mortgage servicing rights	442	316
Assets	2,059	11,526
Measured on non-recurring basis | Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale, at fair value	0	0
Real estate owned	0	0
Mortgage servicing rights	0	0
Assets	0	0
Measured on non-recurring basis | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale, at fair value	0	0
Real estate owned	0	0
Mortgage servicing rights	0	0
Assets	0	0
Measured on non-recurring basis | Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale, at fair value	1,525	10,251
Real estate owned	92	959
Mortgage servicing rights	442	316
Assets	$ 2,059	$ 11,526