Financial Instruments and Fair Value Disclosures (Tables)

12 Months Ended

Dec. 31, 2019

Assets measured at fair value are summarized below (in thousands).

	December 31, 2019
	Fair Value Measurements Using
	Level 1		Level 2		Level 3		Assets at Fair Value
Recurring fair value measurements:
Money market funds	$	135,300	$	—	$	—	$	135,300
Total short-term investments	$	135,300	$	—	$	—	$	135,300

	December 31, 2018
	Fair Value Measurements Using
	Level 1		Level 2		Level 3		Assets at Fair Value		Total Losses for Year Ended ⁽¹⁾
Recurring fair value measurements:
U.S. Treasury bills	$	299,900	$	—	$	—	$	299,900
Money market funds		135,800		—		—		135,800
Short-term investments	$	435,700	$	—	$	—	$	435,700
Nonrecurring fair value measurements:
Impaired assets	$	—	$	—	$	—	$	—	$	27,225

(1)	Represents impairment loss of $27.2 million recognized during 2018 related to a drilling rig whose carrying value was impaired and was subsequently sold. See Note 3.

Fair values and related carrying values of our senior notes (see Note 9) are shown below (in millions).