Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Statement Of Financial Position [Abstract]
Accounts receivable, allowance	$ 6,738	$ 7,270
Accumulated amortization	$ 651,180	$ 583,623
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	475,000	475,000
Common stock, shares issued	86,357	85,459
Common stock, shares outstanding	83,265	83,367
Treasury stock, shares	3,092	2,092