Stockholders' Equity (Details Textual) $ / shares in Units, $ in Thousands	1 Months Ended			3 Months Ended			12 Months Ended
	Mar. 31, 2017 USD ($) $ / shares shares	Jan. 31, 2017 USD ($) $ / shares shares	Jan. 31, 2015	Dec. 31, 2017 USD ($) CustomerAccount shares	Dec. 31, 2016 CustomerAccount	Dec. 31, 2014 CustomerAccount shares	Dec. 31, 2018 USD ($) CustomerAccount $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2014 $ / shares shares
Stockholders Equity Transaction [Line Items]
Remaining number of shares available for repurchase | shares							5,500,000
Repurchase of common stock for employee tax withholdings, shares | shares							159,000	249,000	344,000
Repurchase of common stock for tax withholdings, value							$ 7,400	$ 10,100	$ 13,700
Cash dividends declared per common share | $ / shares							$ 0.84	$ 0.79	$ 0.74
Cash dividend							$ 28,100	$ 26,800	$ 23,800
Stock warrants term										10 years
Stock warrants, exercise price | $ / shares										$ 26.68
Additional number of Residential Customer Accounts converted | CustomerAccount							5,000,000
Client contract costs, net of amortization							$ 37,289
Accumulated amortization expense of client contract incentive related to stock warrants				$ 0			43,051	0
Carrying amount of common stock warrants				$ 9,082			9,082	9,082
Senior Subordinated Convertible Notes 2010
Stockholders Equity Transaction [Line Items]
Common shares from treasury stock delivered to settle conversion obligation in excess of par value | shares	694,240
Carrying value of shares of treasury stock delivered	$ 16,500
Carrying value of treasury stock delivered, per share | $ / shares	$ 23.71
Conversion obligation in excess of par value	$ 28,800
Client contract incentives
Stockholders Equity Transaction [Line Items]
Client contract costs, net of amortization							7,533
Accumulated amortization expense of client contract incentive related to stock warrants							$ 20,833
Conversion Of Comcast Current Residential Customer Accounts
Stockholders Equity Transaction [Line Items]
Issuance of stock warrants | shares						1,900,000				1,900,000
Conversion Of Comcast Current Residential Customer Accounts | First 25 % of Stock Warrants
Stockholders Equity Transaction [Line Items]
Vesting percentage of stock warrants						25.00%
Number of customer account converted | CustomerAccount						500,000
Number of stock warrants vested | shares						500,000
Conversion Of Comcast Current Residential Customer Accounts | Second 25 % of Stock Warrants
Stockholders Equity Transaction [Line Items]
Vesting percentage of stock warrants			25.00%
Vesting date of stock warrants							2015-01
Conversion Of Comcast Current Residential Customer Accounts | Third 25 % of Stock Warrants
Stockholders Equity Transaction [Line Items]
Vesting percentage of stock warrants					25.00%
Number of customer account converted | CustomerAccount					5,500,000
Conversion Of Comcast Current Residential Customer Accounts | Last 25 % of Stock Warrants
Stockholders Equity Transaction [Line Items]
Vesting percentage of stock warrants				25.00%
Number of customer account converted | CustomerAccount				11,000,000
Number of stock warrants vested | shares				400,000
Conversion Of Comcast Acquired Additional Residential Customer Accounts
Stockholders Equity Transaction [Line Items]
Issuance of stock warrants | shares						1,000,000				1,000,000
Comcast
Stockholders Equity Transaction [Line Items]
Issuance of stock warrants | shares						2,900,000				2,900,000
Amount of unvested stock warrants that become vested upon a fundamental change of the Company							50.00%
Beneficial ownership required for potential cash settlement							19.99%
Stock warrants exercised | shares		1,400,000
Stock warrants issued | shares							1,400,000
Stock warrants vested | shares							400,000
Comcast | Common Stock Warrants
Stockholders Equity Transaction [Line Items]
Stock warrants exercised | shares		1,400,000
Common shares from treasury stock delivered to settle conversion obligation in excess of par value | shares		649,221
Fair value of shares of treasury stock delivered		$ 31,500
Carrying value of shares of treasury stock delivered		$ 15,400
Carrying value of treasury stock delivered, per share | $ / shares		$ 23.66
Carrying amount of common stock warrants		$ 16,000
Comcast | Common Stock Warrants | Client contract incentives
Stockholders Equity Transaction [Line Items]
Client contract costs, net of amortization				$ 25,100			$ 25,100	25,100
Accumulated amortization expense of client contract incentive related to stock warrants				$ 9,200			$ 19,800	$ 9,200
SEC Rule 10b5-1 Plan
Stockholders Equity Transaction [Line Items]
Repurchase of common stock, shares | shares							700,000	500,000	300,000
Total amount paid							$ 27,600	$ 20,500	$ 11,600
Weighted-average price per share | $ / shares							$ 39.23	$ 41.10	$ 36.42