Shareholders' Equity Additional Information (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Equity [Abstract]
Repurchase and retirement of common stock			$ 88,877
Repurchase and retirement of common stock (shares)			1,015
Shares remaining available for repurchases (shares)	6,448		6,448
Cash dividend paid during period, per share (in dollars per share)	$ 0.58	$ 0.55	$ 1.74	$ 1.65
Common stock excluded from calculation of diluted earnings per share (shares)	2,892	1,786	2,816	1,736