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, 2018	Dec. 31, 2017	Dec. 31, 2016
Federal Funds Purchased
Short-term Debt [Line Items]
Balance at December 31,	$ 192,064	$ 80,000	$ 32,000
Interest rate at December 31,	2.88%	1.71%	0.85%
Maximum amount outstanding at any month-end during the year	$ 214,165	$ 168,784	$ 133,434
Average amount outstanding during the year	$ 21,338	$ 13,264	$ 8,708
Weighted average interest rate during the year	2.03%	1.42%	0.71%
Securities Sold Under Agreements to Repurchase
Short-term Debt [Line Items]
Balance at December 31,	$ 41,841	$ 44,936	$ 24,354
Interest rate at December 31,	0.26%	0.02%	0.02%
Maximum amount outstanding at any month-end during the year	$ 50,526	$ 44,936	$ 32,872
Average amount outstanding during the year	$ 32,435	$ 28,480	$ 27,535
Weighted average interest rate during the year	0.12%	0.03%	0.03%