Fair Value Measurements (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 771,275,000	$ 633,110,000
Held-to-maturity impairment	0	0
Available-for-sale securities impairment	0	0
Held-to-maturity securities | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	500,130,000	206,161,000
Unrealized gain (loss)	(302,000)	(96,000)
Held-to-maturity securities | Long-term investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	3,859,000
Available-for-sale securities | Auction rate securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cost basis	$ 847,000	$ 1,012,000