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Securities - Narrative (Detail)		3 Months Ended		12 Months Ended
	Jan. 01, 2019 USD ($)	Mar. 31, 2019 USD ($) Security	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) Security
Schedule of Available-for-sale Securities [Line Items]
Percentage of investment portfolio		100.00%		100.00%
Weighted-average period of re-pricing of portfolio		4 years 2 months 12 days		4 years 7 months 12 days
Fair value of securities transferred from held to maturity to available for sale		$ (19,190,000)		$ (42,394,000)	[1]
Fair value of securities transferred from held-to-maturity to available-for-sale		121,202,000		$ 99,857,000
Unrealized losses on securities transferred from held to maturity to available for sale		431,000	$ 0
Impairment recognized		$ 0
U.S. agency and mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Number of securities in an unrealized loss position | Security		66		83
Total number of securities | Security		90		90
Fair value of securities transferred from held to maturity to available for sale		$ (13,861,000)		$ (26,551,000)
Fair value of securities transferred from held-to-maturity to available-for-sale		$ 95,566,000		$ 84,922,000
Obligations of states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Number of securities in an unrealized loss position | Security		26		56
Total number of securities | Security		82		82
Fair value of securities transferred from held to maturity to available for sale		$ (3,816,000)		$ (14,334,000)
Fair value of securities transferred from held-to-maturity to available-for-sale		25,636,000		14,935,000
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Fair value of securities transferred from held to maturity to available for sale		(1,513,000)		$ (1,509,000)
Accounting Standards Update 2017-12
Schedule of Available-for-sale Securities [Line Items]
Fair value of securities transferred from held to maturity to available for sale	$ 23,000,000
Fair value of securities transferred from held-to-maturity to available-for-sale	23,000,000
Unrealized losses on securities transferred from held to maturity to available for sale	$ 431,000	$ 431,000
[1] Derived from audited consolidated financial statements.

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