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
	Aug. 31, 2016	Jul. 31, 2016	Jun. 30, 2016	May 31, 2016	Apr. 30, 2016	Mar. 31, 2016	Feb. 28, 2016	Dec. 31, 2016	Jan. 31, 2016
Equity [Abstract]
Total Number of Shares Purchased	299,931	66,422	330,265	307,798	454,729	269,099	943,983	2,672,227
Average Price Paid Per Share	$ 40.92	$ 41.03	$ 39.96	$ 38.34	$ 38.78	$ 38.27	$ 33.97	$ 37.42
Total Number of Shares Purchased as Part of Publicly Announced Plan	299,931	66,422	330,265	307,798	454,729	269,099	943,983	2,672,227
Maximum Value of Shares that May Yet Be Purchased Under the Plan		$ 12,275	$ 15,000	$ 28,199	$ 40,000	$ 57,632	$ 67,932		$ 100,000