Quarterly report pursuant to Section 13 or 15(d)

Summary of Changes in Stockholders Equity (Parenthetical) (Detail)

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Summary of Changes in Stockholders Equity (Parenthetical) (Detail) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Shareholders Equity [Line Items]        
Cumulative effect of change in accounting principle, taxes     $ 13,079  
Amount per Share of Common Stock $ 0.32 $ 0.29 $ 0.96 $ 0.87
Total     $ 112,524 $ 101,727
First Quarter Dividend        
Shareholders Equity [Line Items]        
Declaration Date     Feb. 23, 2018 Feb. 23, 2017
Record Date     Mar. 08, 2018 Mar. 08, 2017
Payable Date     Mar. 22, 2018 Mar. 20, 2017
Amount per Share of Common Stock     $ 0.32 $ 0.29
Total     $ 37,471 $ 33,912
Second Quarter Dividend        
Shareholders Equity [Line Items]        
Declaration Date     May 25, 2018 May 25, 2017
Record Date     Jun. 08, 2018 Jun. 08, 2017
Payable Date     Jun. 22, 2018 Jun. 22, 2017
Amount per Share of Common Stock     $ 0.32 $ 0.29
Total     $ 37,523 $ 33,904
Third Quarter Dividend        
Shareholders Equity [Line Items]        
Declaration Date     Aug. 23, 2018 Aug. 10, 2017
Record Date     Sep. 04, 2018 Aug. 31, 2017
Payable Date     Sep. 18, 2018 Sep. 13, 2017
Amount per Share of Common Stock     $ 0.32 $ 0.29
Total     $ 37,530 $ 33,911