Stockholders' Equity and Equity Compensation Plans (Details Textual) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2014
Stockholders Equity And Equity Compensation Plans [Line Items]
Remaining number of shares available for repurchase	5,300,000
Repurchase of common stock for employee tax withholdings, shares	100,000	100,000
Repurchase of common stock for tax withholdings, value	$ 4,100	$ 6,200
Cash dividends declared per common share	$ 0.2225	$ 0.21
Cash dividend	$ 7,400	$ 7,100
Stock warrants term				10 years
Stock warrants, exercise price				$ 26.68
Stock-based compensation expense	$ 3,693	$ 4,572
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
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	$ 22,400		$ 19,800
SEC Rule 10b5-1 Plan
Stockholders Equity And Equity Compensation Plans [Line Items]
Repurchase of common stock, shares	200,000	100,000
Total amount paid	$ 9,300	$ 5,700
Weighted-average price per share	$ 37.37	$ 45.78