Accumulated Other Comprehensive Income (Loss) - Changes in Each Component of Accumulated Other Comprehensive Income (Loss), Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ 349,876	$ 302,062	$ 334,835	$ 355,212
Total other comprehensive (loss)	(23,692)	(23,576)	(21,417)	(78,638)
Ending balance	332,745	302,602	332,745	302,602
Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(50,496)	(40,497)	(52,771)	14,675
Other comprehensive loss before reclassifications	(23,692)	(23,626)	(21,417)	(78,794)
Amounts reclassified from accumulated other comprehensive income (loss)	0	(4)	0	(8)
Total other comprehensive (loss)	(23,692)	(23,630)	(21,417)	(78,802)
Ending balance	(74,188)	(64,127)	(74,188)	(64,127)
Defined Benefit Pension Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(5,274)	(5,745)	(5,274)	(5,855)
Other comprehensive loss before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive income (loss)	0	54	0	164
Total other comprehensive (loss)	0	54	0	164
Ending balance	(5,274)	(5,691)	(5,274)	(5,691)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(55,770)	(46,242)	(58,045)	8,820
Other comprehensive loss before reclassifications	(23,692)	(23,626)	(21,417)	(78,794)
Amounts reclassified from accumulated other comprehensive income (loss)	0	50	0	156
Total other comprehensive (loss)	(23,692)	(23,576)	(21,417)	(78,638)
Ending balance	$ (79,462)	$ (69,818)	$ (79,462)	$ (69,818)