Shareholders' Equity - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Aug. 21, 2007
Class of Stock [Line Items]
Cash dividends received	$ 8,114,000	$ 4,372,000
Repurchase of common stock					500,000
Common stock, shares outstanding	30,432,419		30,417,223
Market value of shares repurchased under equity compensation plans	$ 647,000	$ 4,000
2007 Stock Repurchase Plan [Member]
Class of Stock [Line Items]
Common stock, shares outstanding	0	0
Cumulative number of shares repurchased	196,566
Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares outstanding	30,432,419	22,956,323	30,417,223	22,955,963
Company's common stock in lieu of cash to exercise options to purchase shares	16,418	134
Market value of shares repurchased under equity compensation plans	$ 647,000	$ 4,000