Investments and Fair Value Measurements (Fair Value Measurements) (Details) - USD ($)	Mar. 31, 2021	Dec. 31, 2020
Assets:
Debt securities available for sale	$ 102,958,000	$ 91,204,000
Current investment securities at fair value	149,087,000	135,585,000
Long-term investment securities at fair value	35,608,000	34,218,000
Total	463,890,000	472,140,000
Liabilities:
Fair value of contingent liability	926,000	999,000
Total	926,000	999,000
Nonrecurring nonfinancial assets subject to fair value measurements	0	0
Marketable equity securities
Assets:
Total equity securities at fair value	25,389,000	21,155,000
Mutual funds invested in debt securities
Assets:
Total equity securities at fair value	23,240,000	23,226,000
Total equity securities at fair value
Assets:
Total equity securities at fair value	48,629,000	44,381,000
U.S. Government securities
Assets:
Debt securities available for sale	17,976,000
Corporate securities
Assets:
Debt securities available for sale	48,601,000
U.S. mortgage-backed securities
Assets:
Debt securities available for sale	9,626,000
Foreign fixed-income securities
Assets:
Debt securities available for sale	1,224,000
PropTech convertible debt securities
Assets:
Debt securities available for sale	2,500,000
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Long-term investment securities at fair value	0	0
Total	292,744,000	300,210,000
Liabilities:
Fair value of contingent liability	0	0
Total	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Marketable equity securities
Assets:
Total equity securities at fair value	25,389,000	21,155,000
Quoted Prices in Active Markets for Identical Assets (Level 1) | Mutual funds invested in debt securities
Assets:
Total equity securities at fair value	23,240,000	23,226,000
Quoted Prices in Active Markets for Identical Assets (Level 1) | Total equity securities at fair value
Assets:
Total equity securities at fair value	48,629,000	44,381,000
Significant Other Observable Inputs (Level 2)
Assets:
Long-term investment securities at fair value	0	0
Total	135,538,000	137,712,000
Liabilities:
Fair value of contingent liability	0	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	926,000	999,000
Total	926,000	999,000
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:
Current investment securities at fair value	151,587,000	135,585,000
Fair Value, Measurements, Recurring | U.S. Government securities
Assets:
Debt securities available for sale	17,976,000	19,200,000
Fair Value, Measurements, Recurring | Corporate securities
Assets:
Debt securities available for sale	48,601,000	52,434,000
Fair Value, Measurements, Recurring | U.S. mortgage-backed securities
Assets:
Debt securities available for sale	9,626,000	10,484,000
Fair Value, Measurements, Recurring | Commercial paper
Assets:
Debt securities available for sale	23,031,000	9,086,000
Fair Value, Measurements, Recurring | Foreign fixed-income securities
Assets:
Debt securities available for sale	1,224,000
Fair Value, Measurements, Recurring | PropTech convertible debt securities
Assets:
Debt securities available for sale	2,500,000
Fair Value, Measurements, Recurring | Total debt securities available for sale
Assets:
Debt securities available for sale	102,958,000	91,204,000
Fair Value, Measurements, Recurring | Money market funds
Assets:
Cash and cash equivalents	244,115,000	255,294,000
Fair Value, Measurements, Recurring | Commercial paper
Assets:
Cash and cash equivalents	30,467,000	44,397,000
Fair Value, Measurements, Recurring | Commercial paper | Current Restricted Assets
Assets:
Cash and cash equivalents	10,790,000	10,374,000
Fair Value, Measurements, Recurring | Commercial paper | Restricted Assets
Assets:
Cash and cash equivalents	1,907,000	1,907,000
Fair Value, Measurements, Recurring | Certificates of deposit
Assets:
Cash and cash equivalents	2,113,000	2,111,000
Fair Value, Measurements, Recurring | Money market funds securing legal bonds
Assets:
Cash and cash equivalents		535,000
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Current investment securities at fair value	48,629,000	44,381,000
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 paper
Assets:
Debt securities available for sale	0	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) | PropTech convertible debt 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	244,115,000	255,294,000
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
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Assets:
Current investment securities at fair value	102,958,000	91,204,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | U.S. Government securities
Assets:
Debt securities available for sale	17,976,000	19,200,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Corporate securities
Assets:
Debt securities available for sale	48,601,000	52,434,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | U.S. mortgage-backed securities
Assets:
Debt securities available for sale	9,626,000	10,484,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Commercial paper
Assets:
Debt securities available for sale	23,031,000	9,086,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Foreign fixed-income securities
Assets:
Debt securities available for sale	1,224,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | PropTech convertible debt securities
Assets:
Debt securities available for sale	2,500,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Total debt securities available for sale
Assets:
Debt securities available for sale	102,958,000	91,204,000
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	30,467,000	44,397,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Certificates of deposit
Assets:
Cash and cash equivalents	2,113,000	2,111,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Money market funds securing legal bonds
Assets:
Cash and cash equivalents		0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Current 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 paper
Assets:
Debt securities available for sale	0	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) | PropTech convertible debt 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