Commitments and Contingencies - Commitments and Letters of Credit (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Other Commitments [Line Items]
Commitments and letters of credit	$ 2,339,635	$ 1,990,845
Commitments to extend credit
Other Commitments [Line Items]
Commitments and letters of credit	2,251,106	1,910,805
Standby letters of credit
Other Commitments [Line Items]
Commitments and letters of credit	$ 88,529	$ 80,040