Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2015

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

Valuation of marketable securities

	September 30, 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	$	16,112	$	—	$	—	$	16,112

	December 31, 2014
	Quoted Prices in Active Markets for Identical Instruments (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)		Total
Marketable securities	$	15,378	$	—	$	—	$	15,378

Valuation of senior notes

	September 30, 2015				December 31, 2014
	Carrying Amount		Estimated Fair Value		Carrying Amount		Estimated Fair Value
4.875% Senior Notes due 2021	$	498,140	$	550,638	$	497,876	$	566,700
4.625% Senior Notes due 2021	299,689		324,469		299,650		337,222
3.800% Senior Notes due 2022	299,186		307,285		299,109		310,749
3.850% Senior Notes due 2023	$	299,980	$	301,736	$	299,980	$	311,656