Fair Value Measurements - Narrative (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans not covered under loss-share agreements	$ 19,775		$ 29,606
Specific reserve included in allowance for loan losses	168,098	$ 50,814	52,162	$ 145,387	$ 50,059	$ 49,026
Impairment losses on mortgage servicing rights	13,694	3,132	1,836
Changes in fair value, gain	10,876	$ 3,895
Impaired Loans, not covered
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Specific reserve included in allowance for loan losses	$ 8,725		$ 2,258