Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2015

Fair Value Measurements Tables [Abstract]

Fair Value of Long-Term Debt

		December 31, 2015				December 31, 2014
		Carrying		Estimated		Carrying		Estimated
		Amount		Fair Value		Amount		Fair Value
5.45% Notes due March 2015, issued 2008	$	-	$	-	$	100,000	$	101,931
7.31% Notes due March 2016, issued 2009		62,000		63,604		62,000		65,027
4.98% Notes due January 2017, issued 2010		116,000		113,420		116,000		116,240
5.92% Notes due March 2018, issued 2008		200,000		191,985		200,000		203,738
5.75% Notes due December 2018, issued 2013		450,000		111,451		450,000		414,505
7.77% Notes due March 2019, issued 2009		173,000		174,488		173,000		187,105
5.50% Notes due January 2020, issued 2010		207,000		185,052		207,000		201,371
4.51% Notes due October 2020, issued 2010		315,000		258,520		315,000		283,335
5.60% Notes due January 2022, issued 2010		87,000		73,034		87,000		82,581
4.66% Notes due October 2022, issued 2010		35,000		25,558		35,000		30,476
6.125% Notes due October 2024, issued 2014		850,000		206,321		850,000		754,485
5.85% Notes due January 2025, issued 2010		90,000		70,756		90,000		83,876
4.91% Notes due October 2025, issued 2010		175,000		115,911		175,000		147,649
Credit Facility due October 2016		630,000		630,000		518,000		518,000
	$	3,390,000	$	2,220,100	$	3,378,000	$	3,190,319