Shareholders' Equity - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	Aug. 21, 2007
Class of Stock [Line Items]
Cash dividends received	$ 10,236,000	$ 4,770,000	$ 18,350,000	$ 9,142,000
Repurchase of common stock									500,000
Common stock, shares outstanding	30,502,757		30,502,757			30,417,223
Market value of shares repurchased under equity compensation plans			$ 4,695,000	$ 671,000
2007 Stock Repurchase Plan [Member]
Class of Stock [Line Items]
Common stock, shares outstanding	0	0	0	0
Cumulative number of shares repurchased	196,566		196,566
Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares outstanding	30,502,757	23,004,153	30,502,757	23,004,153	30,432,419	30,417,223	22,956,323	22,955,963
Company's common stock in lieu of cash to exercise options to purchase shares	93,755	17,086	119,914	17,220
Market value of shares repurchased under equity compensation plans	$ 3,659,000	$ 667,000	$ 4,695,000	$ 671,000