Bank and Other Notes Payable: (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended		0 Months Ended	1 Months Ended	12 Months Ended				12 Months Ended							1 Months Ended		12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Mar. 15, 2012 Senior Notes	Mar. 31, 2007 Senior Notes	Dec. 31, 2012 Senior Notes	Dec. 31, 2011 Senior Notes	Dec. 31, 2010 Senior Notes	May 02, 2011 Senior Notes	Dec. 31, 2012 Line of Credit	Dec. 31, 2011 Line of Credit	Apr. 25, 2011 Line of Credit	May 02, 2011 Line of Credit Low end of range	Apr. 25, 2011 Line of Credit Low end of range	May 02, 2011 Line of Credit High end of range	Apr. 25, 2011 Line of Credit High end of range	Dec. 31, 2011 Unsecured term loan	Nov. 30, 2011 Unsecured term loan	Dec. 31, 2012 Unsecured term loan	Dec. 08, 2011 Unsecured term loan Low end of range	Dec. 08, 2011 Unsecured term loan High end of range	Dec. 31, 2012 Greeley Note	Dec. 31, 2011 Greeley Note
Bank and other notes payable
Debt issued				$ 950,000												$ 125,000
Interest rate on debt (as a percent)					3.25%
Conversion rate per $1 principal amount					0.0089702
Repurchase and retirement of debt						180,314	18,468
Repurchase of convertible senior notes	180,314	18,191				180,792	18,283
Gain (loss) on early extinguishment of debt	(10,588)	3,661				(1,449)	(489)
Debt, carrying value						437,788															24,027	24,848
Debt, unamortized discount						1,530
Effective interest rate (as a percent)					5.41%	5.41%															6.34%
Debt, fair value						437,788															24,685	26,510
Revolving line of credit								1,500,000			1,500,000
Reference rate for variable interest rate					LIBOR				LIBOR								LIBOR	LIBOR
Term of extension					1 year
Variable interest rate spread (as a percent)									2.00%			1.75%	0.75%	3.00%	1.10%			2.20%	1.95%	3.20%
Interest rate (as a percent)																2.42%		2.57%
Line of credit, average interest rate (as a percent)									2.76%	2.96%
Outstanding borrowings under the line of credit									675,000	290,000
Maximum expanded revolving line of credit									2,000,000
Fair value of outstanding line of credit									675,107	292,366
Fair value of debt																120,019		121,821
Repayment of Senior Notes			$ 439,318