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Short-term Investments | 3 Months Ended
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Mar. 31, 2018
Short Term Investments [Abstract] |
Short-term Investments | (3) Short-term Investments The Company periodically invests a portion of its excess cash in short-term investment instruments. Substantially all of the Company’s short-term investments are in U.S. Treasury securities, and the Company has the ability and intent to hold these investments to maturity. The stated maturity dates of these investments are between three months and one year from the purchase date. These held-to-maturity investments are recorded at amortized cost and included within “Short-term investments” on the accompanying Consolidated Balance Sheets. The fair value of held-to-maturity investments is determined based on quoted market prices in active markets for identical securities (Level 1 inputs). The amortized cost, carrying value, and fair value of held-to-maturity investments at March 31, 2018 were $543.3 million, $543.3 million, and $543.1 million, respectively. The amortized cost, carrying value, and fair value of held-to-maturity investments at December 31, 2017 were $254.9 million, $254.9 million, and $254.8 million, respectively. The gross unrecognized holding gains and losses were not material for each of the three months ended March 31, 2018 and 2017. No other-than-temporary impairments related to these investments have been recognized as of March 31, 2018 and December 31, 2017.

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- Definition The entire disclosure for investments in certain debt and equity securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -URI http://asc.fasb.org/topic&trid=2196928 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=4738149 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 940 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=2176304 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=2324412 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=2209399 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 954 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=2289146 + Details
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