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Available-for-Sale Securities and Fair Value Measurements (Tables) | 6 Months Ended
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Jun. 30, 2019
Schedule of amortized cost and estimated fair values for available-for-sale securities | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | June 30, 2019
​ |  | ​ | ​ |  | Gross |  | Gross |  | ​ | ​
​ | ​ | ​ | ​ | Unrealized | ​ | Unrealized | ​ | Estimated
(In thousands) | ​ | Amortized Cost | ​ | Gains | ​ | Losses | ​ | Fair Value
U.S. government securities | ​ | $ | 49,012 | ​ | $ | 25 | ​ | $ | — | ​ | $ | 49,037
U.S. government agencies | ​ | ​ | 19,919 | ​ | ​ | 8 | ​ | ​ | — | ​ | ​ | 19,927
U.S. commercial paper | ​ |  | 38,747 | ​ |  | — | ​ |  | — | ​ |  | 38,747
Money market funds | ​ |  | 120,581 | ​ |  | — | ​ |  | — | ​ |  | 120,581
Total | ​ | $ | 228,259 | ​ | $ | 33 | ​ | $ | — | ​ | $ | 228,292
​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | December 31, 2018
​ |  | ​ | ​ |  | Gross |  | Gross |  | ​ | ​
​ | ​ | ​ | ​ | Unrealized | ​ | Unrealized | ​ | Estimated
(In thousands) | ​ | Amortized Cost | ​ | Gains | ​ | Losses | ​ | Fair Value
U.S. government securities | ​ | $ | 29,736 | ​ | $ | — | ​ | $ | (3) | ​ | $ | 29,733
U.S. government agencies | ​ | ​ | 4,971 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 4,971
U.S. corporate notes | ​ | ​ | 2,875 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 2,875
U.S. commercial paper | ​ |  | 22,037 | ​ |  | — | ​ |  | — | ​ |  | 22,037
Money market funds | ​ |  | 49,358 | ​ |  | — | ​ |  | — | ​ |  | 49,358
Total | ​ | $ | 108,977 | ​ | $ | — | ​ | $ | (3) | ​ | $ | 108,974
Schedule of available-for-sale securities measured at fair value on a recurring basis | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Estimated Fair Value Measurements as of June 30, 2019 Using:
​ | ​ | Quoted Price in | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Active Markets | ​ | Significant Other | ​ | Significant | ​ | ​ | ​
​ | ​ | for Identical | ​ | Observable | ​ | Unobservable | ​ | ​ | ​
Types of Instruments | ​ | Assets | ​ | Inputs | ​ | Inputs | ​ | ​ | ​
(In thousands) |  | Level 1 |  | Level 2 |  | Level 3 |  | Total
Assets | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
U.S. government securities | ​ | $ | — | ​ | $ | 49,037 | ​ | $ | — | ​ | $ | 49,037
U.S. government agencies | ​ | ​ | — | ​ | ​ | 19,927 | ​ | ​ | — | ​ | ​ | 19,927
U.S. commercial paper | ​ |  | — | ​ |  | 38,747 | ​ |  | — | ​ |  | 38,747
Money market funds | ​ |  | 120,581 | ​ |  | — | ​ |  | — | ​ |  | 120,581
Total assets measured at estimated fair value | ​ | $ | 120,581 | ​ | $ | 107,711 | ​ | $ | — | ​ | $ | 228,292
Debt | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Term B Loan | ​ | $ | — | ​ | $ | 13,750 | ​ | $ | — | ​ | $ | 13,750
2023 Notes | ​ | ​ | — | ​ | ​ | 252,754 | ​ | ​ | — | ​ | ​ | 252,754
2025 Notes | ​ |  | — | ​ |  | 208,181 | ​ |  | — | ​ |  | 208,181
Total fair value of debt | ​ | $ | — | ​ | $ | 474,685 | ​ | $ | — | ​ | $ | 474,685
​ ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Estimated Fair Value Measurements as of December 31, 2018 Using:
Types of Instruments | ​ | Quoted Price in Active Markets for Identical Assets | ​ | Significant Other Observable Inputs | ​ | Significant Unobservable Inputs | ​ | ​ | ​
(In thousands) |  | Level 1 |  | Level 2 |  | Level 3 |  | Total
Assets | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
U.S. government securities | ​ | $ | — | ​ | $ | 29,733 | ​ | $ | — | ​ | $ | 29,733
U.S. government agencies | ​ | ​ | — | ​ | ​ | 4,971 | ​ | ​ | — | ​ | ​ | 4,971
U.S. corporate notes | ​ | ​ | — | ​ | ​ | 2,875 | ​ | ​ | — | ​ | ​ | 2,875
U.S. commercial paper | ​ |  | — | ​ |  | 22,037 | ​ |  | — | ​ |  | 22,037
Money market funds | ​ |  | 49,358 | ​ |  | — | ​ |  | — | ​ |  | 49,358
Total assets measured at estimated fair value | ​ | $ | 49,358 | ​ | $ | 59,616 | ​ | $ | — | ​ | $ | 108,974
Debt | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Term B Loan | ​ | $ | — | ​ | $ | 13,750 | ​ | $ | — | ​ | $ | 13,750
2023 Notes | ​ | ​ | — | ​ | ​ | 258,918 | ​ | ​ | — | ​ | ​ | 258,918
2025 Notes | ​ | ​ | — | ​ | ​ | 230,692 | ​ | ​ | — | ​ | ​ | 230,692
Total fair value of debt | ​ | $ | — | ​ | $ | 503,360 | ​ | $ | — | ​ | $ | 503,360

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- Definition Tabular disclosure of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -URI http://asc.fasb.org/topic&trid=2196928 + Details
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- Definition Tabular disclosure of financial instruments measured at fair value, including those classified in shareholders' equity measured on a recurring or nonrecurring basis. Disclosures include, but are 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. Nonrecurring fair value measurements are those that are required or permitted in the statement of financial position in particular circumstances. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19279-110258 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2C -URI http://asc.fasb.org/extlink&oid=117815213&loc=SL7498348-110258 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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