Reconciliations of Equity - Reconciliations of Total Equity (Parenthetical) (Detail) - $ / shares	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Number of shares of common stock issued under stock option plan	10,779	3,933	16,565	6,680
Number of common stock shares purchased	73,745	71,555	82,045	160,780
Common stock	$ 0.305	$ 0.275	$ 0.610	$ 0.550
Common Stock [Member]
Number of shares of common stock issued under stock option plan	10,779	3,933	16,565	6,680
Additional Paid-in Capital [Member]
Number of shares of common stock issued under stock option plan	10,779	3,933	16,565	6,680
Common Treasury Stock [Member]
Number of common stock shares purchased	73,745	71,555	82,045	160,780
Retained Earnings [Member]
Common stock	$ 0.305	$ 0.275	$ 0.610	$ 0.550