Stockholders' Equity and Equity Compensation Plans (Details Textual) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2014
Stockholders Equity And Equity Compensation Plans [Line Items]
Remaining number of shares available for repurchase	6,100,000
Repurchase of common stock for employee tax withholdings, shares	100,000	200,000
Repurchase of common stock for tax withholdings, value	$ 6,200	$ 6,100
Cash dividends declared per common share	$ 0.21	$ 0.1975
Cash dividend	$ 7,100	$ 6,700
Stock warrants term				10 years
Stock warrants, exercise price				$ 26.68
Stock-based compensation expense	$ 4,572	$ 5,670
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
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	$ 11,800		$ 9,200
SEC Rule 10b5-1 Plan
Stockholders Equity And Equity Compensation Plans [Line Items]
Repurchase of common stock, shares	100,000	100,000
Total amount paid	$ 5,700	$ 5,400
Weighted-average price per share	$ 45.78	$ 42.85