Stockholders' Equity (Details Textual) $ / shares in Units, $ in Millions	1 Months Ended		3 Months Ended				12 Months Ended
	Jan. 31, 2017 shares	Jan. 31, 2015	Dec. 31, 2019 $ / shares shares	Dec. 31, 2017 CustomerAccount shares	Dec. 31, 2016 CustomerAccount	Dec. 31, 2014 CustomerAccount $ / shares shares	Dec. 31, 2019 USD ($) CustomerAccount $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2014 $ / shares shares
Stockholders Equity Transaction [Line Items]
Remaining number of shares available for repurchase			5,000,000.0				5,000,000.0
Repurchase of common stock for employee tax withholdings, shares							117,000	159,000	249,000
Repurchase of common stock for tax withholdings, value | $							$ 5.1	$ 7.4	$ 10.1
Cash dividends declared per common share | $ / shares							$ 0.89	$ 0.84	$ 0.79
Cash dividend | $							$ 29.4	$ 28.1	$ 26.8
Stock warrants term										10 years
Stock warrants, exercise price | $ / shares						$ 26.68				$ 26.68
Additional number of Residential Customer Accounts converted | CustomerAccount							5,000,000
Common Stock Warrants
Stockholders Equity Transaction [Line Items]
Stock warrants, exercise price | $ / shares			$ 26.68				$ 26.68
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						1,000,000.0				1,000,000.0
Comcast
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		400,000				400,000
Stock warrants issued			1,000,000.0				1,000,000.0
Stock warrants vested							0
Fair value of stock warrants | $							$ 24.6
Weighted average price per share | $ / shares			$ 56.12				$ 56.12
Net cash settlement for provision of warrant agreement | $							$ 12.9
Adjustment to additional paid-in capital, warrant issued | $							$ 9.1
Comcast | Common Stock Warrants
Stockholders Equity Transaction [Line Items]
Client contract cost related to stock warrants | $								25.1
Accumulated amortization expense of client contract cost related to stock warrants | $								$ 19.8
SEC Rule 10b5-1 Plan
Stockholders Equity Transaction [Line Items]
Repurchase of common stock, shares							576,000	704,000	500,000
Total amount paid | $							$ 25.5	$ 27.6	$ 20.5
Weighted-average price per share | $ / shares							$ 44.17	$ 39.23	$ 41.10