Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2019

Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]

Valuation of marketable securities

	September 30, 2019
	Quoted Prices in Active Markets for Identical Instruments (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)		Total
Marketable securities	$	29,934	$	—	$	—	$	29,934

	December 31, 2018
	Quoted Prices in Active Markets for Identical Instruments (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)		Total
Marketable securities	$	25,493	$	—	$	—	$	25,493

Valuation of senior notes

	September 30, 2019				December 31, 2018
	Carrying Amount		Estimated Fair Value		Carrying Amount		Estimated Fair Value
Senior Notes	$	3,628,628	$	3,882,575	$	3,130,122	$	3,116,046