Stockholders' Equity (Details Textual) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2014
Stockholders Equity Transaction [Line Items]
Remaining number of shares available for repurchase		4,300,000
Repurchase of common stock for employee tax withholdings, shares		254,000	117,000	159,000
Repurchase of common stock for tax withholdings, value		$ 11.9	$ 5.1	$ 7.4
Cash dividends declared per common share		$ 0.94	$ 0.89	$ 0.84
Cash dividend		$ 30.9	$ 29.4	$ 28.1
Stock warrants term					10 years
Stock warrants, exercise price					$ 26.68
Conversion Of Comcast Current Residential Customer Accounts
Stockholders Equity Transaction [Line Items]
Issuance of stock warrants					1,900,000
Comcast
Stockholders Equity Transaction [Line Items]
Issuance of stock warrants					2,900,000
Stock warrants exercised	400,000		400,000
Fair value of stock warrants		$ 24.6
Weighted average price per share		$ 56.12
Net cash settlement for provision of warrant agreement		$ 12.9
Adjustment to additional paid-in capital, warrant issued		$ 9.1
Stock warrants issued		1,000,000.0
Stock warrants vested		0
Beneficial ownership required for potential cash settlement		19.99%
SEC Rule 10b5-1 Plan
Stockholders Equity Transaction [Line Items]
Repurchase of common stock, shares		624,000	576,000	704,000
Total amount paid		$ 26.3	$ 25.5	$ 27.6
Weighted-average price per share		$ 42.13	$ 44.17	$ 39.23