Stockholders' Equity (Detail) - USD ($) $ in Millions		1 Months Ended	12 Months Ended
	Jun. 23, 2000	Aug. 31, 2007	Dec. 31, 2018	Dec. 31, 2017
Stockholders Equity [Line Items]
Retained earnings from which dividends may not be paid without prior approval, total			$ 21.7	$ 21.1
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			48,011
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			13,008	14,294
Common stock, capital shares reserved for future issuance			143,985
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			313,919