Recent Accounting Pronouncements - Additional Information (Detail) - USD ($)		3 Months Ended		6 Months Ended
	Jan. 01, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings		$ 471,777,000	$ 429,787,000	$ 471,777,000	$ 429,787,000	$ 413,130,000
Unrealized holding gains on available-for-sale portfolio		(6,751,000)	$ 17,535,000	(16,216,000)	$ 26,555,000
Modified retrospective basis through a cumulative-effect adjustment to retained earnings		1,000,000		1,000,000
Amounts reclassified from accumulated other comprehensive income	$ 10,800,000
Accounting Standards Update 2017-12 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings		3,200,000		3,200,000
Reclassification of securities		324,000,000		324,000,000
Unrealized holding gains on available-for-sale portfolio				26,100,000
Accounting Standards Update 2017-12 [Member] | States and Political Subdivisions [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification of securities		$ 447,000,000		447,000,000
Minimum [Member] | Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Anticipated increase in assets and liabilities				125,000,000
Maximum [Member] | Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Anticipated increase in assets and liabilities				$ 140,000,000