Shareholders' Equity - Additional Information (Detail) - USD ($)		1 Months Ended	3 Months Ended
	Jun. 27, 2018	Sep. 30, 2017	Dec. 31, 2018	Jan. 01, 2019	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			31,918,642		30,166,709
Cost of shares repurchased under common stock repurchase program			$ 33,194,000		$ 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			1,262,822
Cost of shares repurchased under common stock repurchase program			$ 1,884,474
Common Stock | Common Stock Repurchase Program
Equity Class Of Treasury Stock [Line Items]
Treasury stock, shares			6,082,677
Cost of shares repurchased under common stock repurchase program			$ 8,970,556
Common Stock | Common Stock Repurchase Program | Subsequent Event
Equity Class Of Treasury Stock [Line Items]
Treasury stock, shares				4,503
Cost of shares repurchased under common stock repurchase program				$ 6,540
Common Stock | Maximum
Equity Class Of Treasury Stock [Line Items]
Stock repurchase program, authorized amount						$ 5,000,000