Stockholders' Equity and Equity Compensation Plans (Details Textual) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	May 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2014	Apr. 30, 2018
Stockholders Equity And Equity Compensation Plans [Line Items]
Remaining number of shares available for repurchase		5,800,000		5,800,000
Repurchase of common stock for employee tax withholdings, shares				200,000	200,000
Repurchase of common stock for tax withholdings, value				$ 7,200	$ 9,400
Cash dividends declared per common share		$ 0.21	$ 0.1975
Cash dividend		$ 7,000	$ 6,700	21,200	20,000
Stock warrants term							10 years
Stock warrants, exercise price							$ 26.68
Stock-based compensation expense		$ 4,600	$ 5,000	$ 14,805	$ 16,659
Performance Shares
Stockholders Equity And Equity Compensation Plans [Line Items]
Performance based awards granted to executive management and certain key employees shares				100,000
Performance Shares | 2018 Performance Awards
Stockholders Equity And Equity Compensation Plans [Line Items]
Vesting maturity date				Mar. 31, 2020
Restricted common stock
Stockholders Equity And Equity Compensation Plans [Line Items]
Vesting period				4 years
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 incentive related to stock warrants				$ 25,100		$ 25,100
Accumulated amortization expense of client contract incentive related to stock warrants				$ 17,100		$ 9,200
Stock Incentive Plan 2005
Stockholders Equity And Equity Compensation Plans [Line Items]
Increase in number of shares approved for issuance under incentive plan	2,700,000
Number of shares approved for issuance under incentive plan	21,400,000							18,700,000
SEC Rule 10b5-1 Plan
Stockholders Equity And Equity Compensation Plans [Line Items]
Repurchase of common stock, shares				400,000	400,000
Total amount paid				$ 16,900	$ 15,600
Weighted-average price per share				$ 42.71	$ 40.54