Allowance for Credit Losses - Allowance for Credit Losses (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Receivables [Abstract]
Allowance for loan losses	$ 138,762	$ 132,862	$ 120,856	$ 116,446	$ 115,443	$ 114,419
Allowance for unfunded letters of credit	4,392		3,596
Total allowance for credit losses	$ 143,154		$ 124,452