Short-Term Borrowings - Summary of Federal Funds Purchased and Other Short-term Borrowings are Included in Federal Home Loan Bank Advances on Consolidated Balance Sheets (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Federal Funds Purchased [Member]
Short-term Debt [Line Items]
Maximum indebtedness		$ 20,000,000
Average balance		$ 116,000
Average rate paid		2.58%
Short Term Borrowings, FHLB advances [Member]
Short-term Debt [Line Items]
Outstanding balance	$ 122,100,000	$ 188,600,000
Maximum indebtedness	192,700,000	225,300,000
Average balance	$ 92,267,000	$ 113,520,000
Average rate paid	2.51%	2.07%
Interest rate on balance	2.51%	2.45%