Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2016

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

Valuation of marketable securities

	March 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	$	17,933	$	—	$	—	$	17,933

	December 31, 2015
	Quoted Prices in Active Markets for Identical Instruments (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)		Total
Marketable securities	$	16,895	$	—	$	—	$	16,895

Valuation of senior notes

	March 31, 2016				December 31, 2015
	Carrying Amount		Estimated Fair Value		Carrying Amount		Estimated Fair Value
$500 million, 4.875% Senior Notes due 2021	$	496,153	$	545,237	$	495,951	$	542,078
$300 million, 4.625% Senior Notes due 2021	298,466		325,027		298,396		319,620
$300 million, 3.800% Senior Notes due 2022	297,617		315,934		297,535		303,595
$300 million, 3.850% Senior Notes due 2023	298,190		314,925		$	298,136	$	302,468
$500 million, 3.550% Senior Notes due 2026	$	495,451	$	517,907