Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2018

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

Valuation of marketable securities

	September 30, 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	$	28,514	$	—	$	—	$	28,514

	December 31, 2017
	Quoted Prices in Active Markets for Identical Instruments (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)		Total
Marketable securities	$	25,706	$	—	$	—	$	25,706

Valuation of senior notes

	September 30, 2018				December 31, 2017
	Carrying Amount		Estimated Fair Value		Carrying Amount		Estimated Fair Value
Senior Notes	$	3,129,327	$	3,119,604	$	2,632,390	$	2,728,167