Financial Instruments and Fair Value Disclosures (Tables)

6 Months Ended

Jun. 30, 2019

Fair Value Disclosures [Abstract]

Fair Value, Assets Measured on Recurring Basis

Certain of our assets and liabilities are required to be measured at fair value on a recurring basis in accordance with GAAP. Assets measured at fair value are summarized below (in thousands).

	June 30, 2019
	Fair Value Measurements Using
	Level 1		Level 2		Level 3		Assets at Fair Value
U.S. Treasury bills	$	149,945	$	—	$	—	$	149,945
Money market funds		134,319		—		—		134,319
Total short-term investments	$	284,264	$	—	$	—	$	284,264

	Year ended December 31, 2018
	Fair Value Measurements Using
	Level 1		Level 2		Level 3		Assets at Fair Value
U.S. Treasury bills	$	299,849	$	—	$	—	$	299,849
Money market funds		135,822		—		—		135,822
Total short-term investments	$	435,671	$	—	$	—	$	435,671

Fair Values and Related Carrying Values of Our Debt Instruments

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

	June 30, 2019				December 31, 2018
	Fair Value		Carrying Value		Fair Value		Carrying Value
3.45% Senior Notes due 2023	$	210.0	$	249.5	$	185.0	$	249.5
7.875% Senior Notes due 2025		475.0		497.1		415.0		496.8
5.70% Senior Notes due 2039		335.0		497.3		305.0		497.2
4.875% Senior Notes due 2043		465.0		748.9		416.3		748.9