Investments in Equity Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Investments in equity securities	$ 1,867,838	$ 1,533,517
Charter
Debt and Equity Securities, FV-NI [Line Items]
Investments in equity securities	1,858,883	1,526,984
Other investments
Debt and Equity Securities, FV-NI [Line Items]
Investments in equity securities	$ 8,955	$ 6,533