Stockholders' Equity and Equity Compensation Plans (Details Textual) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Mar. 31, 2015	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Feb. 28, 2015
Stockholders Equity And Equity Compensation Plans [Line Items]
Increase in number of shares authorized for repurchase							7,500,000
Number of shares authorized for repurchase under stock repurchase program							42,500,000
Shares repurchased				300,000	200,000
Total amount paid				$ 7.0	$ 5.2
Weighted-average price per share				$ 27.06	$ 27.26
Remaining number of shares available for repurchase		7,300,000		7,300,000
Repurchase of common stock for employee tax withholdings, shares				300,000	200,000
Repurchase of common stock for tax withholdings, value				$ 8.0	$ 6.7
Cash dividends declared per common share		$ 0.1750	$ 0.1575	$ 0.5250	$ 0.4650
Cash dividend		$ 5.7	$ 5.5	$ 17.2	$ 16.0
Stock warrants term						10 years
Stock warrants, exercise price						$ 26.68
Stock-based compensation expense		$ 5.3	$ 4.5	$ 15.8	$ 12.3
Performance Shares
Stockholders Equity And Equity Compensation Plans [Line Items]
Performance based awards granted to executive management shares				100,000
Stock-based compensation expense				3 years
Restricted common stock
Stockholders Equity And Equity Compensation Plans [Line Items]
Stock-based compensation expense		4 years		4 years
Migration Of Comcast Current Residential Customer Accounts
Stockholders Equity And Equity Compensation Plans [Line Items]
Number of stock warrants vesting				1,000,000
Comcast Corporation
Stockholders Equity And Equity Compensation Plans [Line Items]
Issuance of stock warrants						2,900,000
Client contract incentive related to stock warrants				$ 7.3
Amortization expense of client contract incentive related to stock warrants				$ 1.6
Stock warrants exercised				0
ASR Agreement
Stockholders Equity And Equity Compensation Plans [Line Items]
Shares repurchased	1,300,000
Total amount paid	$ 40.0
Accelerated share repurchase agreement, total amount paid	$ 50.0