Marketable Investment Securities - Schedule of Marketable Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Debt securities	$ 1,870,818	$ 2,200,650
Equity securities	64,887	90,976
Total marketable investment securities	1,935,705	2,291,626
Less: Restricted marketable investment securities	10,597	9,474
Marketable investment securities, at fair value	1,925,108	2,282,152
Corporate bonds
Schedule of Investments [Line Items]
Debt securities	1,470,693	1,735,653
Other debt securities
Schedule of Investments [Line Items]
Debt securities	$ 400,125	$ 464,997