Financial Instruments and Fair Value Disclosures (Tables)

3 Months Ended

Mar. 31, 2019

Fair Value Disclosures [Abstract]

Fair Value, Assets Measured on Recurring Basis

	March 31, 2019
	Fair Value Measurements Using
	Level 1		Level 2		Level 3		Assets at Fair Value
Recurring fair value measurements:
Short-term investments	$	359,027	$	—	$	—	$	359,027

	December 31, 2018
	Fair Value Measurements Using
	Level 1		Level 2		Level 3		Assets at Fair Value
Recurring fair value measurements:
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).

	March 31, 2019				December 31, 2018
	Fair Value		Carrying Value		Fair Value		Carrying Value
3.45% Senior Notes due 2023	$	214.4	$	249.5	$	185.0	$	249.5
7.875% Senior Notes due 2025		482.5		497.0		415.0		496.8
5.70% Senior Notes due 2039		336.2		497.2		305.0		497.2
4.875% Senior Notes due 2043		474.4		748.9		416.3		748.9