Shareholders' Equity - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Aug. 13, 2015
Shareholders' Equity [Line Items]
Authorized and unissued common shares available for issuance	300,000,000	300,000,000
Common shares purchase price as percentage of fair market value	95.00%
Maximum value of shares purchased as percentage of employee compensation	10.00%
Common stock issued	$ 404,000	$ 388,000	$ 391,000
Dividend Reinvestment and Stock Purchase Plan [Member]
Shareholders' Equity [Line Items]
Authorized and unissued common shares available for issuance	3,300,000			3,300,000
Employee Stock Purchase Plan [Member]
Shareholders' Equity [Line Items]
Number of shares available for purchase, maximum	500,000
Shares issued related to dividend reinvestment and stock purchase plan	24,000	32,000
Common stock issued	$ 404,000	$ 388,000