Stockholders' Equity (Detail) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Aug. 31, 2007	Dec. 31, 2020	Dec. 31, 2019
Stockholders Equity [Line Items]
Retained earnings from which dividends may not be paid without prior approval, total		$ 21.2	$ 21.4
Employee Stock Purchase
Stockholders Equity [Line Items]
Number of options issued or activity	0
Employee eligibility period		1 year
Employees to buy stock under options granted		85.00%
Options are vested when granted and will expire no later than		27 months
Common stock, capital shares reserved for future issuance		25,739
Dividend Reinvestment And Stock Purchase Plan [Member]
Stockholders Equity [Line Items]
Employees to buy stock under options granted		95.00%
Shares of common stock purchased		14,566	13,316
Common stock, capital shares reserved for future issuance		76,538
Stockholder Purchase Plan [Member]
Stockholders Equity [Line Items]
Number of options issued or activity		0
Options are vested when granted and will expire no later than		3 months
Common stock, capital shares reserved for future issuance		183,348