Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 2017

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

Valuation of marketable securities

	June 30, 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	$	23,355	$	—	$	—	$	23,355

	December 31, 2016
	Quoted Prices in Active Markets for Identical Instruments (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)		Total
Marketable securities	$	20,462	$	—	$	—	$	20,462

Valuation of senior notes

	June 30, 2017				December 31, 2016
	Carrying Amount		Estimated Fair Value		Carrying Amount		Estimated Fair Value
Senior Notes	$	1,888,062	$	1,997,080	$	1,887,019	$	1,977,510