Marketable Investment Securities (Details 2) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Unrealized Gains (Losses) on Available-For-Sale Securities
Amortized Cost	$ 1,384,955	$ 1,384,955	$ 684,943
Unrealized Gains	246	246	0
Unrealized Losses	(325)	(325)	(334)
Debt securities	1,384,876	1,384,876	684,609
Available-for-sale Securities, Debt Maturities [Abstract]
Available-for-sale Securities, Debt Maturities, Next Rolling Twelve Months, Fair Value	1,080,000	1,080,000
Available For Sale Securities Debt Maturities After Next Rolling Twelve Months Fair Value	396,300	396,300
Corporate bonds
Unrealized Gains (Losses) on Available-For-Sale Securities
Amortized Cost	1,075,236	1,075,236	542,861
Unrealized Gains	246	246	0
Unrealized Losses	(306)	(306)	(288)
Debt securities	1,075,176	1,075,176	542,573
Available-for-sale Securities, Debt Maturities [Abstract]
Fair value of investments	92,800	92,800		$ 0
Unrealized gain on investments	23,300	23,100
Other debt securities
Unrealized Gains (Losses) on Available-For-Sale Securities
Amortized Cost	309,719	309,719	142,082
Unrealized Gains	0	0	0
Unrealized Losses	(19)	(19)	(46)
Debt securities	$ 309,700	$ 309,700	$ 142,036