Shareholders' Equity - Summary of Activities in Common Stock (Detail) - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity [Abstract]
Issuance of shares pursuant to stock compensation plan, shares	1,113	827	1,118
Purchases of common stock, Shares	(44)	(882)	(5)
Employee stock plan purchases, Shares	51	47	43
Shares issued for acquisitions, Shares			2,149
Issuance of shares pursuant to stock compensation plan	$ 49,845	$ 23,527	$ 23,160
Purchases of common stock, Amount	(6,613)	(112,699)	(645)
Employee stock plan purchases, Amount	$ 7,044	$ 6,236	4,671
Shares issued for acquisitions, Amount			$ 364,333