Shareholders' Equity Additional Information (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
Equity [Abstract]
Repurchase and retirement of common stock			$ 246,931
Repurchase and retirement of common stock (shares)			2,069,000
Stock Repurchase Program, Number of Shares Authorized to be Repurchased	4,367,000		4,367,000
Cash dividend paid during period, per share (in dollars per share)	$ 0.6	$ 0.58	$ 1.8	$ 1.74
Common stock excluded from calculation of diluted earnings per share (shares)	1,785,000	2,892,000	1,713,000	2,816,000