Shareholders' Equity - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017	Aug. 21, 2007
Class of Stock [Line Items]
Cash dividends received	$ 7,011,000	$ 13,507,000	$ 25,361,000	$ 22,649,000
Repurchase of common stock (in shares)									500,000
Common stock, shares outstanding	30,512,187		30,512,187			30,417,223
Stock repurchase program cumulative number Of shares repurchased during the deriod			0	0
Market value of shares repurchased under equity compensation plans			$ 4,842,000	$ 1,124,000
2007 Stock Repurchase Plan
Class of Stock [Line Items]
Cumulative number of shares repurchased	196,566		196,566
Common Stock
Class of Stock [Line Items]
Common stock, shares outstanding	30,512,187	30,417,818	30,512,187	30,417,818	30,502,757	30,417,223	23,004,153	22,955,963
Company's common stock in lieu of cash to exercise options to purchase shares (in shares)	3,820	11,630	123,734	28,850
Market value of shares repurchased under equity compensation plans	$ 147,000	$ 453,000	$ 4,842,000	$ 1,124,000