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Fair Value Measurements Fair Value Measurements (Tables) | 12 Months Ended
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Dec. 31, 2017
Fair Value Disclosures [Abstract] |
Marketable Securities [Table Text Block] | Fair Value of Instruments Measured and Recognized at Fair Value The following tables summarize the levels of fair value measurements of our financial assets:
|  | Fair Value Measurements
|  |  |  | (in thousands) |  |
|  | December 31, 2017 |  | Level 1 |  | Level 2 |  | Level 3
Money market funds |  | $ | 360,746 |  |  | $ | 360,746 |  |  | $ | — |  |  | $ | — |
Municipal bonds |  | 1,960 |  |  | — |  |  | 1,960 |  |  | — |
Total |  | $ | 362,706 |  |  | $ | 360,746 |  |  | $ | 1,960 |  |  | $ | — |
|  | Fair Value Measurements
|  |  |  | (in thousands) |  |
|  | December 31, 2016 |  | Level 1 |  | Level 2 |  | Level 3
Money market funds |  | $ | 283,322 |  |  | $ | 283,322 |  |  | $ | — |  |  | $ | — |
Municipal bonds |  | 1,851 |  |  | — |  |  | 1,851 |  |  | — |
Total |  | $ | 285,173 |  |  | $ | 283,322 |  |  | $ | 1,851 |  |  | $ | — |
Available-for-sale Securities [Table Text Block] | The following is a summary of our available-for-sale marketable securities:
|  | Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Estimated Fair Value
|  |  |  | (in thousands) |  |
December 31, 2017 |  |  |  |  |  |  |  |
Municipal bonds |  | $ | 1,965 |  |  | $ | — |  |  | $ | (5 | ) |  | $ | 1,960 |
December 31, 2016 |  |  |  |  |  |  |  |  |  |  |  |
Municipal bonds |  | $ | 1,854 |  |  | $ | — |  |  | $ | (3 | ) |  | $ | 1,851 |
Investments Classified by Contractual Maturity Date [Table Text Block] | As of December 31, 2017, the contractual maturities of our marketable securities were as follows:
|  | Amortized Cost |  | Estimated Fair Value
|  | (in thousands)
Less than one year |  | $ | 1,860 |  |  | $ | 1,856 |
One to five years |  | 105 |  |  | 104 |
Total |  | $ | 1,965 |  |  | $ | 1,960 |

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- Definition Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=75019621&loc=d3e62557-112803 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27198-111563 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=75019621&loc=d3e62586-112803 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 + Details
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- Definition Tabular disclosure of assets measured at fair value measured on a recurring or nonrecurring basis. Includes, but is not limited to, fair value measurements recorded and the reasons for the measurements, level within the fair value hierarchy in which the fair value measurements are categorized and transfers between levels 1 and 2. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19190-110258 + Details
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- Definition Tabular disclosure of maturities of an entity's investments as well as any other information pertinent to the investments. + References No definition available. + Details
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