Shareholders' Equity - Additional Information (Detail) - USD ($)		1 Months Ended	3 Months Ended
	Jun. 27, 2018	Sep. 30, 2017	Sep. 30, 2018	Nov. 09, 2018	Oct. 01, 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			30,655,820			30,166,709
Cost of shares repurchased under common stock repurchase program			$ 31,310,000			$ 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			233,649
Cost of shares repurchased under common stock repurchase program			$ 346,898
Stock repurchase program, remaining balance			$ 2,913,918
Common Stock | Subsequent Event
Equity Class Of Treasury Stock [Line Items]
Stock repurchase program, remaining balance				$ 1,654,010
Common Stock | Common Stock Repurchase Program
Equity Class Of Treasury Stock [Line Items]
Treasury stock, shares			4,819,855
Cost of shares repurchased under common stock repurchase program			$ 7,086,082
Common Stock | Common Stock Repurchase Program | Subsequent Event
Equity Class Of Treasury Stock [Line Items]
Treasury stock, shares					844,571
Cost of shares repurchased under common stock repurchase program					$ 1,259,908
Common Stock | Maximum
Equity Class Of Treasury Stock [Line Items]
Stock repurchase program, authorized amount							$ 5,000,000