Shareholders' Equity - Additional Information (Detail) - USD ($)		1 Months Ended	3 Months Ended
	Jun. 27, 2018	Sep. 30, 2017	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2016
Equity Class Of Treasury Stock [Line Items]
Options outstanding			0
Warrants outstanding			0
Other potentially dilutive securities outstanding			0
Treasury stock, shares			30,422,171	30,166,709
Cost of shares repurchased under common stock repurchase program			$ 30,963,000	$ 30,583,000
Common Stock
Equity Class Of Treasury Stock [Line Items]
Stock repurchase program, authorized amount		$ 10,000,000
Stock repurchase program, additional authorized repurchase amount		$ 5,000,000
Stock repurchase program, expiration date	Dec. 31, 2018	Jun. 30, 2018
Number of common shares repurchased			255,462
Cost of shares repurchased under common stock repurchase program			$ 379,947
Stock repurchase program, remaining balance			$ 3,260,816
Common Stock | Common Stock Repurchase Program
Equity Class Of Treasury Stock [Line Items]
Treasury stock, shares			4,586,206
Cost of shares repurchased under common stock repurchase program			$ 6,739,184
Common Stock | Maximum
Equity Class Of Treasury Stock [Line Items]
Stock repurchase program, authorized amount					$ 5,000,000