Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 2015

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

Valuation of marketable securities

	Quoted Prices in Active Markets for Identical Instruments (Level 1)				Significant Other Observable Inputs (Level 2)				Significant Unobservable Inputs (Level 3)				Total
	June 30, 2015		December 31, 2014		June 30, 2015		December 31, 2014		June 30, 2015		December 31, 2014		June 30, 2015		December 31, 2014
Marketable securities	$	16,957	$	15,378	$	—	$	—	$	—	$	—	$	16,957	$	15,378

Valuation of senior notes

	June 30, 2015				December 31, 2014
	Carrying Amount		Estimated Fair Value		Carrying Amount		Estimated Fair Value
4.875% Senior Notes due 2021	$	498,052	$	543,999	$	497,876	$	566,700
4.625% Senior Notes due 2021	299,676		323,712		299,650		337,222
3.800% Senior Notes due 2022	299,160		303,234		299,109		310,749
3.850% Senior Notes due 2023	$	299,980	$	300,559	$	299,980	$	311,656