Stockholders' Equity and Equity Compensation Plans (Details Textual) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2014
Stockholders Equity And Equity Compensation Plans [Line Items]
Remaining number of shares available for repurchase	5,100,000		5,100,000
Repurchase of common stock for employee tax withholdings, shares	4,000	5,000	112,000	154,000
Repurchase of common stock for tax withholdings, value	$ 200	$ 200	$ 4,700	$ 7,200
Cash dividends declared per common share	$ 0.2225	$ 0.21
Cash dividend	$ 7,300	$ 7,000	22,000	21,000
Stock warrants term						10 years
Stock warrants, exercise price						$ 26.68
Stock-based compensation expense	$ 4,800	$ 4,600	$ 13,295	$ 14,805
Performance Shares
Stockholders Equity And Equity Compensation Plans [Line Items]
Performance based awards granted to executive management and certain key employees shares			100,000
Vesting maturity date			Mar. 31, 2021
Vesting period			2 years
Restricted common stock
Stockholders Equity And Equity Compensation Plans [Line Items]
Vesting period			4 years
Common Stock Warrants
Stockholders Equity And Equity Compensation Plans [Line Items]
Stock warrants, exercise price	$ 26.68		$ 26.68
Comcast
Stockholders Equity And Equity Compensation Plans [Line Items]
Issuance of stock warrants						2,900,000
Stock warrants issued	1,400,000		1,400,000
Stock warrants vested			400,000
Comcast | Common Stock Warrants
Stockholders Equity And Equity Compensation Plans [Line Items]
Client contract costs related to stock warrants					$ 25,100
Accumulated amortization expense of client contract costs related to stock warrants					$ 19,800
SEC Rule 10b5-1 Plan
Stockholders Equity And Equity Compensation Plans [Line Items]
Repurchase of common stock, shares	92,000	138,000	485,000	395,000
Total amount paid	$ 4,700	$ 5,500	$ 20,600	$ 16,900
Weighted-average price per share	$ 51.09	$ 39.87	$ 42.37	$ 42.71