Allowance for Loan Losses - Change in Allowance for Loan Losses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes in the allowance for loan losses
Balance at the beginning of period	$ 37,066	$ 35,569	$ 35,606
Provision charged to operations	16,435	18,040	15,512
Loans charged off	(22,396)	(21,170)	(20,317)
Recoveries of loans previously charged off	4,621	4,627	4,768
Balance at the end of period	$ 35,726	$ 37,066	$ 35,569