Debt and Credit Agreements (Details 2) (USD $)	12 Months Ended			1 Months Ended	12 Months Ended		1 Months Ended				12 Months Ended	1 Months Ended						1 Months Ended					12 Months Ended			12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Jul. 31, 2001 7.33% weighted-average fixed rate notes item	Dec. 31, 2013 7.33% weighted-average fixed rate notes	Dec. 31, 2013 7.33% weighted-average fixed rate notes Greater than Q	Jul. 31, 2001 7.33% weighted-average fixed rate notes 7.33% Weighted-Average Fixed Rate Notes Tranche 1 due July 2011	Jul. 31, 2001 7.33% weighted-average fixed rate notes 7.33% Weighted-Average Fixed Rate Notes Tranche 2 due July 2013	Jul. 31, 2001 7.33% weighted-average fixed rate notes 7.33% Weighted-Average Fixed Rate Notes Tranche 3 due July 2016	Jul. 31, 2008 6.51% weighted-average fixed rate notes item	Dec. 31, 2013 6.51% weighted-average fixed rate notes Greater than Q	Jul. 31, 2008 6.51% weighted-average fixed rate notes 6.51% Weighted-Average Fixed Rate Notes Tranche 1 due July 2018	Jul. 31, 2008 6.51% weighted-average fixed rate notes 6.51% Weighted-Average Fixed Rate Notes Tranche 2 due July 2020	Jul. 31, 2008 6.51% weighted-average fixed rate notes 6.51% Weighted-Average Fixed Rate Notes Tranche 3 due July 2023	Dec. 31, 2008 9.78% notes item	Dec. 31, 2013 9.78% notes	Dec. 31, 2012 9.78% notes	Dec. 31, 2010 5.58% weighted-average fixed rate notes item	Dec. 31, 2010 5.58% weighted-average fixed rate notes 5.58% Weighted-Average Fixed Rate Notes Tranche 1 due January 2021	Dec. 31, 2010 5.58% weighted-average fixed rate notes 5.58% Weighted-Average Fixed Rate Notes Tranche 2 due January 2023	Dec. 31, 2010 5.58% weighted-average fixed rate notes 5.58% Weighted-Average Fixed Rate Notes Tranche 3 due January 2026	Dec. 31, 2013 Revolving Credit Facility	Dec. 31, 2013 Revolving Credit Facility Q	Dec. 30, 2013 Revolving Credit Facility	May 31, 2012 Revolving Credit Facility	Dec. 31, 2013 Revolving Credit Facility Greater than
Debt
Number of institutional investors that purchased debt in a private placement offering				7						41					4			8
Number of tranches				3						3								3
Principal				$ 170,000,000			$ 75,000,000	$ 75,000,000	$ 20,000,000	$ 425,000,000		$ 245,000,000	$ 100,000,000	$ 80,000,000	$ 67,000,000			$ 175,000,000	$ 88,000,000	$ 25,000,000	$ 62,000,000
Term							10 years	12 years	15 years			10 years	12 years	15 years	10 years				10 years	12 years	15 years
Coupon (as a percent)							7.26%	7.36%	7.46%			6.44%	6.54%	6.69%	9.78%	9.78%	9.78%		5.42%	5.59%	5.80%
Asset coverage ratio						1.75					1.75															1.75
Annual coverage ratio						2.8					2.8															2.8
Repayments of debt	895,000,000	263,000,000	355,000,000		150,000,000
Number of trailing quarters used to calculate annual coverage ratio						4					4												4
Borrowing base																						2,300,000,000	2,300,000,000
Line of credit																								1,400,000,000	900,000,000
Increase of maximum borrowing capacity																									500,000,000
Debt issuance costs																						2,800,000
Period to reduce outstanding debt in equal monthly installments to the adjusted credit line available																							6 months
Remaining unamortized costs under amended credit facility																						$ 10,600,000	$ 10,600,000