Fair Value Measurements - Assets and Liabilities Measured on Non-Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans measured for impairment based on the fair value of the underlying collateral	$ 0	$ 0
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans measured for impairment based on the fair value of the underlying collateral	0	0
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans measured for impairment based on the fair value of the underlying collateral	6,737,316	6,219,008
Measured on non-recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans measured for impairment based on the fair value of the underlying collateral		1,063
Mortgage servicing rights	443	681
Assets	443	1,744
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 measured for impairment based on the fair value of the underlying collateral		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 measured for impairment based on the fair value of the underlying collateral		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 measured for impairment based on the fair value of the underlying collateral		1,063
Mortgage servicing rights	443	681
Assets	$ 443	$ 1,744