Shareholders' Equity - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Nov. 12, 2019
Class of Stock [Line Items]
Cash dividends received	$ 63,419,000	$ 32,669,000	$ 26,432,000
Repurchase of common stock (in shares)					1,525,000
Stock repurchase plan percentage of common stock					5.00%
Common stock, shares outstanding (in shares)	29,727,214	30,523,824
Stock repurchase plan, amount of shares repurchased	$ 27,069,000	$ 5,108,000	3,969,000
Market value of shares repurchased under equity compensation plans	$ 736,000	$ 5,108,000	$ 1,486,000
Stock options
Class of Stock [Line Items]
Company's common stock in lieu of cash to exercise options to purchase shares (in shares)	12,488	115,954	59,025
Other share-based awards
Class of Stock [Line Items]
Company's common stock in lieu of cash to exercise options to purchase shares (in shares)	12,058		15,242	45,964
Maximum
Class of Stock [Line Items]
Amount available to dividend to holding company	$ 111,492,000
20019 Stock Repurchase Plan
Class of Stock [Line Items]
Cumulative number of shares repurchased (in shares)	858,717
Stock repurchase plan, number of shares repurchased (in shares)		0
2007 Stock Repurchase Plan
Class of Stock [Line Items]
Stock repurchase plan, number of shares repurchased (in shares)		0	26,966
Stock repurchase plan, amount of shares repurchased			$ 968,000
Common Stock
Class of Stock [Line Items]
Stock repurchase plan, number of shares repurchased (in shares)	883,263	131,196	104,989
Common stock, shares outstanding (in shares)	29,727,214	30,523,824	30,417,223	22,955,963
Stock repurchase plan, amount of shares repurchased	$ 15,746,000	$ 2,339,000	$ 1,677,000
Market value of shares repurchased under equity compensation plans	$ 735,837		$ 5,108,000	$ 3,001,000