Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Accounts receivable, allowance	$ 9,741	$ 9,801
Accumulated depreciation	109,846	97,889
Accumulated amortization	$ 108,283	$ 90,685
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized (in shares)	10,000,000	10,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	200,000,000	200,000,000
Common stock, shares issued (in shares)	48,809,000	48,411,000
Common stock, shares outstanding (in shares)	46,914,000	47,481,000
Treasury stock (in shares)	1,895,000	930,000