Shareholders' Equity - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Nov. 12, 2019	Dec. 31, 2016
Class of Stock [Line Items]
Cash dividends received	$ 32,669,000	$ 26,432,000	$ 19,236,000
Repurchase of common stock (in shares)				1,525,000
Stock repurchase plan percentage of common stock				5.00%
Common stock, shares outstanding (in shares)	30,523,824	30,417,223
Stock repurchase plan, number of shares repurchased (in shares)	0	196,566	0
Shares repurchased, fair value		$ 968,000
Market value of shares repurchased under equity compensation plans	$ 5,108,000	$ 1,486,000	$ 2,225,000
Stock options
Class of Stock [Line Items]
Company's common stock in lieu of cash to exercise options to purchase shares (in shares)	115,954	59,025	92,410
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)	15,242	45,964	14,980
Maximum
Class of Stock [Line Items]
Amount available to dividend to holding company	$ 131,000,000
2007 Stock Repurchase Plan
Class of Stock [Line Items]
Cumulative number of shares repurchased (in shares)	0
Stock repurchase plan, number of shares repurchased (in shares)		26,966	26,966
Common Stock
Class of Stock [Line Items]
Common stock, shares outstanding (in shares)	30,523,824	30,417,223	22,955,963		22,867,802
Stock repurchase plan, number of shares repurchased (in shares)	131,196	104,989	107,390
Market value of shares repurchased under equity compensation plans	$ 5,108,000	$ 3,001,000	$ 3,854,000