Shareholders' Equity - Additional Information (Detail) - USD ($)			3 Months Ended								12 Months Ended
	Mar. 21, 2018	Feb. 20, 2018	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	May 03, 2016
Class of Stock [Line Items]
Stock dividend declared, percent							15.00%					15.00%
Dividend declared per common share (in dollars per share)			$ 0.31	$ 0.31	$ 0.31	$ 0.31	$ 0.27	$ 0.27	$ 0.27	$ 0.27	$ 1.24	$ 1.08	$ 0.94
Cash dividends paid - common shares, total											$ 43,100,000	$ 37,500,000	$ 33,200,000
Preferred shares, authorized (in shares)			30,000,000								30,000,000
Preferred shares, par value (in dollars per share)			$ 1.00								$ 1.00
Preferred shares, issued (in shares)			0				0				0	0
Preferred shares, outstanding (in shares)			0				0				0	0
Common shares repurchased on open market (in shares)												530,882	756,252
Common shares repurchased on open market, total cost												$ 31,700,000	$ 45,200,000
2016 Repurchase Authorization [Member]
Class of Stock [Line Items]
Remaining number of shares available under repurchase authorization, Value			$ 53,300,000								$ 53,300,000
Common shares repurchased on open market (in shares)												530,882
Common shares repurchased on open market, total cost												$ 31,700,000
Maximum [Member] | 2016 Repurchase Authorization [Member]
Class of Stock [Line Items]
Total number of shares authorized for purchase, value														$ 150,000,000.0
Stock Dividend [Member]
Class of Stock [Line Items]
Stock dividend declared, percent		15.00%
Additional stock issued as dividend (in shares)	4,397,520	4,397,520
Before 2016 Adjustment Stock Dividend [Member]
Class of Stock [Line Items]
Dividend declared per common share (in dollars per share)													$ 0.94