CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 259,251	$ 265,636	$ 252,521
Other comprehensive income (loss):
Foreign currency translation adjustments	19,808	(5,488)	(20,410)
Comprehensive income	$ 279,059	$ 260,148	$ 232,111