Reconciliations of Equity - Reconciliations of Total Equity (Parenthetical) (Detail) - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Number of shares of common stock issued under stock option plan	0	40,435	16,565	47,115
Number of common stock shares purchased	53,058		135,103	160,780
Common stock	$ 0.305	$ 0.275	$ 0.915	$ 0.825
Common Stock [Member]
Number of shares of common stock issued under stock option plan	0	40,435	16,565	47,115
Additional Paid-in Capital [Member]
Number of shares of common stock issued under stock option plan	0	40,435	16,565	47,115
Common Treasury Stock [Member]
Number of common stock shares purchased	53,058		135,103	160,780
Retained Earnings [Member]
Common stock	$ 0.305	$ 0.275	$ 0.915	$ 0.825