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Marketable Securities (Notes)
 12 Months Ended
Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]  
Marketable Securities Marketable Securities
At December 31, 2017, marketable securities consisted of the following (in thousands):
Amortized Cost Unrealized Gains Unrealized Losses Aggregate Fair Value 
U.S. treasury debt securities $3,083 $— $(8)$3,075 
U.S. corporate debt securities 13,350 — (61)13,289 
Money market funds 49,452 — — 49,452 
$65,885 $— $(69)$65,816 
Included in cash and cash equivalents $49,452 $— $— $49,452 
Included in marketable securities $16,433 $— $(69)$16,364 
At December 31, 2016, marketable securities consisted of the following (in thousands):
Amortized Cost Unrealized Gains Unrealized Losses Aggregate Fair Value 
U.S. treasury debt securities $3,503 $— $(5)$3,498 
U.S. corporate debt securities 7,943 1 (7)7,937 
Money market funds 43,496 — — 43,496 
$54,942 $1 $(12)$54,931 
Included in cash and cash equivalents $43,496 $— $— $43,496 
Included in marketable securities $11,446 $1 $(12)$11,435 
The following table presents gross unrealized losses and fair values for those marketable securities that were in an unrealized loss position as of December 31, 2017, aggregated by investment category and the length of time that individual securities have been in a continuous loss position (in thousands):
As of December 31, 2017 
Less than 12 months 12 months or greater 
Fair Value Unrealized Loss Fair Value Unrealized Loss 
U.S. treasury debt securities $1,976 $(7)$1,099 $(1)
U.S. corporate debt securities 13,289 (61)— — 
Total $15,265 $(68)$1,099 $(1)
We do not believe any of the unrealized losses represented an other-than-temporary impairment based on our evaluation of available evidence, which includes our intent as of December 31, 2017 to hold these investments until the cost basis is recovered.