Marketable Investment Securities Marketable Investment Securities (Details 3) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Proceeds from Sale of Available-for-sale Securities [Abstract] (Deprecated 2018-01-31)
Proceeds from Sale of Available-for-sale Securities	$ 150,900,000	$ 0	$ 150,900,000	$ 31,000,000.0
Available-for-sale Securities, Gross Realized Gains	0	0	0	2,800,000
Equity securities
Gain (Loss) on Securities [Line Items]
Available-for-sale Securities, Equity Securities, Current					$ 87,100,000
Trading Securities, Equity		23,900,000		23,900,000	52,500,000
Available-for-sale Equity Securities, Amortized Cost Basis					97,500,000
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax					7,900,000
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax					$ 18,400,000
Other Nonoperating Income (Expense) [Member] | Equity securities
Gain (Loss) on Securities [Line Items]
Other-than-temporary impairment loss on available-for-sale securities				3,300,000
Gain (Loss) on Investments [Member] | Equity securities
Gain (Loss) on Securities [Line Items]
Trading Securities, Change in Unrealized Holding Gain (Loss)	$ 19,700,000	$ 19,900,000	$ 25,300,000	$ 19,900,000