Investments (Narrative) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 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	$ 358,500,000	$ 458,000,000
Gross unrealized losses on available-for-sale investments	10,696,000		$ 7,298,000
Private Equity Funds [Member]
Investment [Line Items]
Equity securities accounted for at net asset value	10,000,000
Other Income (Expense) [Member]
Investment [Line Items]
Realized gains on the sales of available-for-sale investments	100,000	500,000
Realized losses on the sales of available-for-sale investments	1,100,000	$ 100,000
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	$ 7,100,000
Cost method investments			$ 18,600,000