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Stock Buy Back Plans - Number of Shares Purchased and Maximum Value of Shares That yet be Purchased Under Plan (Detail) - USD ($)
$ / shares in Units, $ in Thousands
 1 Months Ended 12 Months Ended
Sep. 30, 2015
Aug. 31, 2015
Jul. 31, 2015
Feb. 28, 2015
Dec. 31, 2015
Nov. 30, 2015
Oct. 31, 2015
Jun. 30, 2015
May. 31, 2015
Apr. 30, 2015
Mar. 31, 2015
Jan. 31, 2015
Equity [Abstract]                        
Total Number of Shares Purchased 81,229 198,923 114,352 341,020 735,524              
Average Price Paid Per Share $ 37.43 $ 39.97 $ 38.58 $ 39.35 $ 39.19              
Total Number of Shares Purchased as Part of Publicly Announced Plan 81,229 198,923 114,352 341,020 735,524              
Maximum Value of Shares that May Yet Be Purchased Under the Plan $ 28,154 $ 31,194 $ 39,145 $ 43,577   $ 28,154 $ 28,154 $ 43,577 $ 43,577 $ 43,577 $ 43,577 $ 56,977