Debt - Summary of Information Relating to Securities Sold Under Agreements to Repurchase and Federal Funds Purchased (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Federal Funds Purchased
Short-term Debt [Line Items]
Balance at December 31	$ 80,000	$ 32,000	$ 115,000
Interest rate at December 31	1.71%	0.85%	0.65%
Maximum amount outstanding at any month-end during the year	$ 168,784	$ 133,434	$ 130,000
Average amount outstanding during the year	$ 13,264	$ 8,708	$ 22,734
Weighted average interest rate during the year	1.42%	0.71%	0.45%
Securities Sold Under Agreements to Repurchase
Short-term Debt [Line Items]
Balance at December 31	$ 44,936	$ 24,354	$ 36,234
Interest rate at December 31	0.02%	0.02%	0.02%
Maximum amount outstanding at any month-end during the year	$ 44,936	$ 32,872	$ 40,140
Average amount outstanding during the year	$ 28,480	$ 27,535	$ 31,293
Weighted average interest rate during the year	0.03%	0.03%	0.03%