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 ($) CustomerAccount shares	Dec. 31, 2016 USD ($) CustomerAccount	Dec. 31, 2014 CustomerAccount shares	Dec. 31, 2017 USD ($) CustomerAccount $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 $ / shares shares
Stockholders Equity Transaction [Line Items]
Remaining number of shares available for repurchase				6,200,000			6,200,000
Repurchase of common stock for employee tax withholdings, shares							249,000	344,000	265,000
Repurchase of common stock for tax withholdings, value | $							$ 10,100	$ 13,700	$ 8,100
Cash dividends declared per common share | $ / shares							$ 0.79	$ 0.74	$ 0.70
Cash dividend | $							$ 26,800	$ 23,800	$ 22,900
Stock warrants term										10 years
Stock warrants, exercise price | $ / shares										$ 26.68
Additional number of Residential Customer Accounts converted | CustomerAccount							5,000,000
Carrying amount of common stock warrants | $				$ 9,082	$ 16,007		$ 9,082	16,007
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	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
Conversion Of Comcast Current Residential Customer Accounts
Stockholders Equity Transaction [Line Items]
Issuance of stock warrants						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						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				400,000
Conversion Of Comcast Acquired Additional Residential Customer Accounts
Stockholders Equity Transaction [Line Items]
Issuance of stock warrants						1,000,000				1,000,000
Comcast Corporation
Stockholders Equity Transaction [Line Items]
Issuance of stock warrants						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%			19.99%
Stock warrants exercised		1,400,000
Stock warrants issued				1,400,000			1,400,000
Stock warrants vested							400,000
Comcast Corporation | Common stock Warrants
Stockholders Equity Transaction [Line Items]
Client contract incentive related to stock warrants | $				$ 25,100	$ 16,000		$ 25,100	16,000
Accumulated amortization expense of client contract incentive related to stock warrants | $							$ 9,200	$ 4,300
Stock warrants exercised		1,400,000
Common shares from treasury stock delivered to settle conversion obligation in excess of par value		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
SEC Rule 10b5-1 Plan
Stockholders Equity Transaction [Line Items]
Repurchase of common stock, shares							500,000	300,000	300,000
Total amount paid | $							$ 20,500	$ 11,600	$ 6,900
Weighted-average price per share | $ / shares							$ 41.10	$ 36.42	$ 27.06
ASR Agreement
Stockholders Equity Transaction [Line Items]
Repurchase of common stock, shares									1,600,000
Weighted-average price per share | $ / shares									$ 31.64
Accelerated share repurchase agreement, total amount paid | $									$ 50,000