Stockholders' Equity - Additional Information (Detail) - USD ($)	1 Months Ended		3 Months Ended												12 Months Ended
	Jan. 31, 2019	Sep. 30, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Oct. 31, 2016	Jan. 31, 2016
Class of Stock [Line Items]
Common stock, shares authorized			110,000,000				110,000,000								110,000,000	110,000,000
Common stock entitles															One
Cash dividends declared per common share			$ 0.42	$ 0.42	$ 0.42	$ 0.42	$ 0.33	$ 0.33	$ 0.33	$ 0.33	$ 0.26	$ 0.26	$ 0.26	$ 0.26	$ 1.68	$ 1.32	$ 1.04
Share Repurchase Program [Member]
Class of Stock [Line Items]
Shares repurchase program period		15 months
Shares repurchase program authorized		$ 100,000,000						$ 100,000,000											$ 25,000,000
Shares repurchase program additional authorized amount																		$ 50,000,000
Commencement date		2017-10
Share Repurchase Program [Member] | Subsequent Event [Member]
Class of Stock [Line Items]
Shares repurchase program period	2 years
Shares repurchase program authorized	$ 100,000,000
Commencement date	2019-04
Common Stock Non-Voting [Member]
Class of Stock [Line Items]
Common stock, shares authorized			10,000,000				10,000,000								10,000,000	10,000,000
Common Stock Voting [Member]
Class of Stock [Line Items]
Common stock, shares authorized			110,000,000				110,000,000								110,000,000	110,000,000