Short-term Investments	12 Months Ended
Dec. 31, 2018
Short Term Investments [Abstract]
Short-term Investments	(4) 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 all short-term investments have stated maturity dates between three months and one year from the purchase date. All short-term investments are included within “Short-term investments” on the accompanying Consolidated Balance Sheets. The fair value of the Company’s short-term investments is determined based on quoted market prices in active markets for identical securities (Level 1 inputs). In the fourth quarter of 2018, the Company began to redeem certain short-term investments prior to their stated maturity dates. As a result, all short-term investments were reclassified from held-to-maturity to available-for-sale. As of December 31, 2018, all short-term investments are classified as available-for-sale and reported at fair value. The amortized cost and fair value of available-for-sale investments at December 31, 2018 were $466.6 million and $466.2 million, respectively. The gross unrecognized holding losses accumulated in other comprehensive loss were $0.4 million for the year ended December 31, 2018. The gross unrecognized holding gains accumulated in other comprehensive loss were not material for the year-ended December 31, 2018. No other-than-temporary impairments related to these available-for-sale investments have been recognized as of December 31, 2018. As of December 31, 2017, all short-term investments were classified as held-to-maturity and reported at amortized cost. 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 the years ended December 31, 2017 and 2016. No other-than-temporary impairments related to these held-to-maturity investments have been recognized as of December 31, 2017.

Short-term Investments

12 Months Ended

Dec. 31, 2018

(4) 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 all short-term investments have stated maturity dates between three months and one year from the purchase date. All short-term investments are included within “Short-term investments” on the accompanying Consolidated Balance Sheets. The fair value of the Company’s short-term investments is determined based on quoted market prices in active markets for identical securities (Level 1 inputs).

In the fourth quarter of 2018, the Company began to redeem certain short-term investments prior to their stated maturity dates. As a result, all short-term investments were reclassified from held-to-maturity to available-for-sale. As of December 31, 2018, all short-term investments are classified as available-for-sale and reported at fair value. The amortized cost and fair value of available-for-sale investments at December 31, 2018 were $466.6 million and $466.2 million, respectively. The gross unrecognized holding losses accumulated in other comprehensive loss were $0.4 million for the year ended December 31, 2018. The gross unrecognized holding gains accumulated in other comprehensive loss were not material for the year-ended December 31, 2018. No other-than-temporary impairments related to these available-for-sale investments have been recognized as of December 31, 2018.

As of December 31, 2017, all short-term investments were classified as held-to-maturity and reported at amortized cost. 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 the years ended December 31, 2017 and 2016. No other-than-temporary impairments related to these held-to-maturity investments have been recognized as of December 31, 2017.