Investments (Narrative) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Investment [Line Items]
Average remaining maturities for short-term available for sale investments			7 months
Average remaining maturities for long-term available for sale investments			2 years
Proceeds from available-for-sale of investments	$ 7,500,000	$ 116,400,000	$ 442,400,000	$ 678,500,000
Gross unrealized losses on available-for-sale investments	9,200,000		9,200,000		$ 7,300,000
Private Equity Funds [Member]
Investment [Line Items]
Alternative Investment	11,200,000		11,200,000
Other Income (Expense) [Member]
Investment [Line Items]
Realized gains on sales of available-for-sale investments	0	0	100,000	700,000
Debt Securities, Available-for-sale, Realized Loss	100,000	$ 200,000	1,500,000	$ 400,000
Level 3 [Member]
Investment [Line Items]
Adjustments resulting from observable price changes	0
Other Assets [Member]
Investment [Line Items]
Direct investments in privately-held companies	$ 8,400,000		$ 8,400,000
Cost method investments					$ 18,600,000