Allowance for Credit Losses - Allowance for Credit Losses (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Receivables [Abstract]
Allowance for loan losses	$ 161,853	$ 155,105	$ 151,859	$ 144,963	$ 138,762	$ 120,856
Allowance for unfunded letters of credit	2,917		4,436
Total allowance for credit losses	$ 164,770		$ 156,295