Investments and Fair Value Measurements (Fair Value Measurements) (Details) - USD ($)	Jun. 30, 2020	Dec. 31, 2019
Assets:
Total equity securities at fair value	$ 66,990,000	$ 91,977,000
Debt securities available for sale	54,885,000	83,445,000
Investment securities at fair value	88,643,000	129,641,000
Long-term investment securities at fair value	33,232,000	45,781,000
Total	633,648,000	533,133,000
Liabilities:
Fair value of contingent liability	1,317,000	3,147,000
Fair value of derivatives embedded within convertible debt		4,999,000
Total	1,317,000	8,146,000
Nonrecurring nonfinancial assets subject to fair value measurements	0	0
Marketable equity securities
Assets:
Total equity securities at fair value	11,091,000	23,819,000
Mutual funds invested in debt securities
Assets:
Total equity securities at fair value	22,667,000	22,377,000
Total equity securities at fair value
Assets:
Total equity securities at fair value	33,758,000	46,196,000
U.S. Government securities
Assets:
Debt securities available for sale	12,754,000
Corporate securities
Assets:
Debt securities available for sale	37,145,000
U.S. mortgage-backed securities
Assets:
Debt securities available for sale	4,984,000
Commercial mortgage-backed securities
Assets:
Debt securities available for sale	2,000
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Long-term investment securities at fair value	0	0
Total	489,962,000	354,386,000
Liabilities:
Fair value of contingent liability	0	0
Fair value of derivatives embedded within convertible debt		0
Total	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Marketable equity securities
Assets:
Total equity securities at fair value	11,091,000	23,819,000
Quoted Prices in Active Markets for Identical Assets (Level 1) | Mutual funds invested in debt securities
Assets:
Total equity securities at fair value	22,667,000	22,377,000
Quoted Prices in Active Markets for Identical Assets (Level 1) | Total equity securities at fair value
Assets:
Total equity securities at fair value	33,758,000	46,196,000
Significant Other Observable Inputs (Level 2)
Assets:
Long-term investment securities at fair value	0	0
Total	110,454,000	132,966,000
Liabilities:
Fair value of contingent liability	0	0
Fair value of derivatives embedded within convertible debt		0
Total	0	0
Significant Other Observable Inputs (Level 2) | Marketable equity securities
Assets:
Total equity securities at fair value	0	0
Significant Other Observable Inputs (Level 2) | Mutual funds invested in debt securities
Assets:
Total equity securities at fair value	0	0
Significant Other Observable Inputs (Level 2) | Total equity securities at fair value
Assets:
Total equity securities at fair value	0	0
Significant Unobservable Inputs (Level 3)
Assets:
Long-term investment securities at fair value	0	0
Total	0	0
Liabilities:
Fair value of contingent liability	1,317,000	3,147,000
Fair value of derivatives embedded within convertible debt		4,999,000
Total	1,317,000	8,146,000
Significant Unobservable Inputs (Level 3) | Marketable equity securities
Assets:
Total equity securities at fair value	0	0
Significant Unobservable Inputs (Level 3) | Mutual funds invested in debt securities
Assets:
Total equity securities at fair value	0	0
Significant Unobservable Inputs (Level 3) | Total equity securities at fair value
Assets:
Total equity securities at fair value	0	0
Fair Value, Measurements, Recurring
Assets:
Investment securities at fair value	88,643,000	129,641,000
Fair Value, Measurements, Recurring | U.S. Government securities
Assets:
Debt securities available for sale	12,754,000	14,660,000
Fair Value, Measurements, Recurring | Corporate securities
Assets:
Debt securities available for sale	37,145,000	54,413,000
Fair Value, Measurements, Recurring | U.S. mortgage-backed securities
Assets:
Debt securities available for sale	4,984,000	6,816,000
Fair Value, Measurements, Recurring | Commercial mortgage-backed securities
Assets:
Debt securities available for sale	2,000	382,000
Fair Value, Measurements, Recurring | Commercial paper
Assets:
Debt securities available for sale		5,887,000
Fair Value, Measurements, Recurring | Index-linked U.S. bonds
Assets:
Debt securities available for sale		779,000
Fair Value, Measurements, Recurring | Foreign fixed-income securities
Assets:
Debt securities available for sale		508,000
Fair Value, Measurements, Recurring | Total debt securities available for sale
Assets:
Debt securities available for sale	54,885,000	83,445,000
Fair Value, Measurements, Recurring | Money market funds
Assets:
Cash and cash equivalents	455,669,000	307,655,000
Fair Value, Measurements, Recurring | Commercial paper
Assets:
Cash and cash equivalents	53,463,000	47,328,000
Fair Value, Measurements, Recurring | Commercial paper | Current Restricted Assets
Assets:
Cash and cash equivalents	6,310,000	4,423,000
Fair Value, Measurements, Recurring | Commercial paper | Restricted Assets
Assets:
Cash and cash equivalents	1,907,000	3,160,000
Fair Value, Measurements, Recurring | Certificates of deposit
Assets:
Cash and cash equivalents	2,106,000	2,193,000
Fair Value, Measurements, Recurring | Money market funds securing legal bonds
Assets:
Cash and cash equivalents	535,000	535,000
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Investment securities at fair value	33,758,000	46,196,000
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Government securities
Assets:
Debt securities available for sale	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate securities
Assets:
Debt securities available for sale	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. mortgage-backed securities
Assets:
Debt securities available for sale	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Commercial mortgage-backed securities
Assets:
Debt securities available for sale	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Commercial paper
Assets:
Debt securities available for sale		0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Index-linked U.S. bonds
Assets:
Debt securities available for sale		0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Foreign fixed-income securities
Assets:
Debt securities available for sale		0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Total debt securities available for sale
Assets:
Debt securities available for sale	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds
Assets:
Cash and cash equivalents	455,669,000	307,655,000
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Commercial paper
Assets:
Cash and cash equivalents	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Certificates of deposit
Assets:
Cash and cash equivalents	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds securing legal bonds
Assets:
Cash and cash equivalents	535,000	535,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Assets:
Investment securities at fair value	54,885,000	83,445,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | U.S. Government securities
Assets:
Debt securities available for sale	12,754,000	14,660,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Corporate securities
Assets:
Debt securities available for sale	37,145,000	54,413,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | U.S. mortgage-backed securities
Assets:
Debt securities available for sale	4,984,000	6,816,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Commercial mortgage-backed securities
Assets:
Debt securities available for sale	2,000	382,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Commercial paper
Assets:
Debt securities available for sale		5,887,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Index-linked U.S. bonds
Assets:
Debt securities available for sale		779,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Foreign fixed-income securities
Assets:
Debt securities available for sale		508,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Total debt securities available for sale
Assets:
Debt securities available for sale	54,885,000	83,445,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Money market funds
Assets:
Cash and cash equivalents	0	0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Commercial paper
Assets:
Cash and cash equivalents	53,463,000	47,328,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Certificates of deposit
Assets:
Cash and cash equivalents	2,106,000	2,193,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Money market funds securing legal bonds
Assets:
Cash and cash equivalents	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Investment securities at fair value	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | U.S. Government securities
Assets:
Debt securities available for sale	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Corporate securities
Assets:
Debt securities available for sale	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | U.S. mortgage-backed securities
Assets:
Debt securities available for sale	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Commercial mortgage-backed securities
Assets:
Debt securities available for sale	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Commercial paper
Assets:
Debt securities available for sale		0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Index-linked U.S. bonds
Assets:
Debt securities available for sale		0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Foreign fixed-income securities
Assets:
Debt securities available for sale		0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Total debt securities available for sale
Assets:
Debt securities available for sale	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Money market funds
Assets:
Cash and cash equivalents	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Commercial paper
Assets:
Cash and cash equivalents	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Certificates of deposit
Assets:
Cash and cash equivalents	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Money market funds securing legal bonds
Assets:
Cash and cash equivalents	$ 0	$ 0