Employee Benefit Plans - Additional Information (Detail) - USD ($)		1 Months Ended		12 Months Ended
		May 31, 2017	Feb. 28, 2013	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant awards		1,500,000		1,000,000
Maximum number of shares of common stock		200,000		100,000
Incentive compensation program expiration				2022-05
Fair value of grants recognition period	[1]			5 years
Additional grant awards		500,000
Vested of common stock				1
Issuance of common stock				1
Dividend equivalents charged against retained earnings when common stock dividends were declared				$ 749,000	$ 542,000	$ 445,000
Compensation expense				$ 1,777,000	1,350,000	1,426,000
Employer matching contribution percent				50.00%
Employee compensation				3.00%
Contributions, net of forfeitures				$ 295,000	$ 282,000	$ 268,000
Restricted Stock Units [Member]
Schedule Of Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Restricted stock awarded				124,650	94,250	86,600
Termination of grant				10 years
RSUs vest starting period from date of grant				1 year
Annual rate of vest of total number of RSUs				20.00%
Number of outstanding RSUs				573,575	448,925	429,775	400,375
Number of vested RSUs				289,020	225,385	221,819	202,344
Restricted Stock Units [Member] | 2012 Incentive Program [Member] | Employees And Directors [Member]
Schedule Of Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Restricted stock awarded			35,000
Restricted Stock Units [Member] | 2004 Omnibus Incentive Compensation Plan [Member]
Schedule Of Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Fair value of grants recognition period				5 years
Compensation expense				$ 1,752,000	$ 1,328,000	$ 1,418,000
Weighted average period				3 years
Number of outstanding RSUs				573,575
Number of vested RSUs				289,020
Intrinsic value of outstanding RSUs				$ 16,869,000
Intrinsic value of vested RSUs				8,500,000
Restricted Stock Units [Member] | 2004 Incentive Plan [Member]
Schedule Of Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Unrecognized compensation cost				$ 5,121,000
Supplemental Employee Retirement Plan [Member]
Schedule Of Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Executive compensation				10.00%
Distribution from supplemental plan				$ 0
Supplemental Employee Retirement Plan [Member] | Former Officers [Member]
Schedule Of Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Distribution from supplemental plan					$ 278,000	$ 469,000
Tranche One [Member]
Schedule Of Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Incentive compensation program vesting percentage				20.00%

[1] five-year