Long-Term Debt (Details) (USD $)	12 Months Ended			1 Months Ended	12 Months Ended	1 Months Ended			12 Months Ended	36 Months Ended		1 Months Ended			12 Months Ended	1 Months Ended	12 Months Ended					12 Months Ended					12 Months Ended					12 Months Ended							12 Months Ended		12 Months Ended									1 Months Ended				12 Months Ended			12 Months Ended		12 Months Ended				12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Nov. 30, 1998 State and Local Government Municipal Bond Issues	Dec. 31, 2011 State and Local Government Municipal Bond Issues	Apr. 30, 2002 Electric Utility Development Loans	Dec. 31, 2011 Electric Utility Development Loans	Dec. 31, 2010 Electric Utility Development Loans	Dec. 31, 2011 Minnesota Economic Development State Loans	Dec. 31, 2010 Minnesota Economic Development State Loans	Dec. 31, 2011 Mesabi Nugget Kobe Iron Nugget, LLC (Kobe)	Jan. 31, 2012 7 3/8% senior notes, due 2012	Dec. 31, 2011 7 3/8% senior notes, due 2012	Dec. 31, 2010 7 3/8% senior notes, due 2012	Dec. 31, 2011 7 3/8% senior notes, due 2012 Maximum	Jun. 30, 2009 5.125% convertible senior notes, due 2014	Dec. 31, 2011 5.125% convertible senior notes, due 2014 year day	Dec. 31, 2010 5.125% convertible senior notes, due 2014	Dec. 31, 2009 5.125% convertible senior notes, due 2014	Dec. 31, 2011 6 3/4% senior notes, due 2015	Dec. 31, 2010 6 3/4% senior notes, due 2015	Dec. 31, 2011 6 3/4% senior notes, due 2015 Debt redemption on or after April 1, 2011	Dec. 31, 2011 6 3/4% senior notes, due 2015 Debt redemption on or after April 1, 2012	Dec. 31, 2011 6 3/4% senior notes, due 2015 Debt redemption on or after April 1, 2013	Dec. 31, 2011 7 3/4% senior notes, due 2016	Dec. 31, 2010 7 3/4% senior notes, due 2016	Dec. 31, 2011 7 3/4% senior notes, due 2016 Debt redemption after April 15, 2012	Dec. 31, 2011 7 3/4% senior notes, due 2016 Debt redemption on or after April 1, 2013	Dec. 31, 2011 7 3/4% senior notes, due 2016 Debt redemption on or after April 1, 2014	Dec. 31, 2011 7 5/8% senior notes, due 2020	Dec. 31, 2010 7 5/8% senior notes, due 2020	Dec. 31, 2011 7 5/8% senior notes, due 2020 Debt redemption prior to March 15, 2013 offering	Dec. 31, 2011 7 5/8% senior notes, due 2020 Debt redemption after March 15, 2015	Dec. 31, 2011 7 5/8% senior notes, due 2020 Debt redemption on or after March 15, 2016	Dec. 31, 2011 7 5/8% senior notes, due 2020 Debt redemption on or after March 15, 2017	Dec. 31, 2011 7 5/8% senior notes, due 2020 Debt redemption on or after March 15, 2018	Dec. 31, 2011 Other secured obligations	Dec. 31, 2010 Other secured obligations	Dec. 31, 2011 Senior secured revolving credit facility, due 2016 month	Sep. 29, 2011 Senior secured revolving credit facility, due 2016	Dec. 31, 2011 Senior secured revolving credit facility, due 2016 Scenario actual	Dec. 31, 2011 Senior secured revolving credit facility, due 2016 Minimum	Dec. 31, 2011 Senior secured revolving credit facility, due 2016 Minimum Scenario forecasted	Dec. 31, 2011 Senior secured revolving credit facility, due 2016 Minimum LIBOR	Dec. 31, 2011 Senior secured revolving credit facility, due 2016 Minimum Prime	Dec. 31, 2011 Senior secured revolving credit facility, due 2016 Maximum	Dec. 31, 2011 Senior secured revolving credit facility, due 2016 Maximum Scenario forecasted	Dec. 31, 2011 Senior secured revolving credit facility, due 2016 Maximum LIBOR	Dec. 31, 2011 Senior secured revolving credit facility, due 2016 Maximum Prime	Jan. 31, 2012 Term Loan maturing in 2016	Jan. 31, 2012 Term Loan maturing in 2016 Minimum	Jan. 31, 2012 Term Loan maturing in 2016 Maximum	Dec. 31, 2011 Stand-by letter of credit Electric Utility Development Loans	Dec. 31, 2011 Senior secured revolving credit facility after amendment Scenario forecasted	Dec. 31, 2011 Senior secured revolving credit facility after amendment Mesabi Nugget	Dec. 31, 2011 Term loan facilities Mesabi Nugget	Dec. 31, 2010 Mesabi Nugget Loan Participation	Dec. 31, 2011 Mesabi Nugget Loan Participation	Dec. 31, 2010 Mesabi Nugget Loan Participation Kobe Iron Nugget, LLC (Kobe)	Dec. 31, 2009 Mesabi Nugget Loan Participation Kobe Iron Nugget, LLC (Kobe) agreement	Dec. 31, 2011 Mesabi Nugget Loan Participation Mesabi Nugget	Dec. 31, 2010 Mesabi Nugget Loan Participation Mesabi Nugget	Dec. 31, 2010 Mesabi Nugget Loan Participation Mesabi Nugget Kobe Iron Nugget, LLC (Kobe)	Dec. 31, 2011 Mesabi Nugget Loan Participation Mesabi Nugget Kobe Iron Nugget, LLC (Kobe)
Long Term Debt
Total debt	$ 2,380,100,000	$ 2,386,821,000											$ 700,000,000	$ 700,000,000			$ 287,500,000	$ 287,500,000		$ 500,000,000	$ 500,000,000				$ 500,000,000	$ 500,000,000				$ 350,000,000	$ 350,000,000						$ 42,600,000	$ 49,321,000																										$ 4,500,000
Less current maturities	444,078,000	8,924,000																																																														901,000
Long-term debt	1,936,022,000	2,377,897,000
Stated interest rate (as a percent)					7.30%		8.10%		3.50%			7.375%	7.375%	7.375%		5.125%	5.125%	5.125%	5.125%	6.75%	6.75%				7.75%	7.75%				7.625%	7.625%
Debt issued																287,500,000
Maximum borrowing capacity of credit facility before renewal																																								924,000,000
Maximum borrowing capacity of credit facility																																							1,100,000,000	1,100,000,000															85,000,000	240,000,000
Additional amount by which facility size can be increased																																							125,000,000	125,000,000
Quarterly interest rate percentage																																																		2.00%	1.25%	3.75%
Unpaid principal balance	682,461,000																																																	158,000,000
Repayments of Debt	7,740,000	346,963,000	1,690,557,000		7,100,000							279,700,000
Maximum amount of debt that could be tendered															350,000,000
Expenses recorded in January 2012 related to tender premium, unamortized financing fees and tender expenses												13,900,000
Reference rate, description																																												LIBOR	Prime			LIBOR	Prime
Interest rate added to the base rate (as a percent)																																												1.00%				2.00%	1.00%
Unused commitment fee (as a percent)																																										0.25%				0.45%
Percent of eligible accounts receivable included in maximum availability of credit facility calculation																																							85.00%
Percent of eligible inventories included in maximum availability of credit facility calculation																																							65.00%
Availability on the senior secured revolver																																							1,100,000,000
Outstanding letters of credit and other obligations which reduce availability																																							16,500,000
Financial covenant, minimum liquidity																																									1,600,000,000		150,000,000
Financial covenant, outstanding																																										850,000,000
Numerator portion of consolidated interest coverage ratio																																									4.92		2.5
Denominator portion of consolidated interest coverage ratio																																									1		1
Numerator portion of debt leverage ratio																																									2.36						5.00
Denominator portion of debt leverage ratio																																									1.00						1.00
Numerator portion of threshold debt leverage ratio after which payment restrictions occur																																															3.50
Denominator portion of threshold debt leverage ratio after which payment restrictions occur																																															1.00
Anticipated covenant compliance period (in months)																																																						twelve months
Proceeds from issuance of common stock and convertible senior notes																678,800,000
Debt instrument maturity period (in years)																	5
Convertible debt, shares, basis for conversion																	56.9801
Convertible debt, principal amount, basis for conversion																	1,000
Convertible debt, shares issued																	16,381,779
Number of consecutive trading days during which the closing price of the entity's common stock must exceed the conversion price for at least 20 or more trading days in order for the notes to be redeemable																	20
Number of days within 30 consecutive trading days in which the closing price of the entity's common stock must exceed the conversion price for the notes to be redeemable (in days)																	30
Minimum sales price as percentage of conversion price considered for redemption of notes																	130.00%
Conversion price per share (in dollars per share)																	$ 17.55
Redemption price of debt instrument (as a percent)																	100.00%					103.375%	101.688%	100.00%			103.875%	101.938%	100.00%			107.625%	103.813%	102.542%	101.271%	100.00%
Proceeds from debt	10,103,000	556,553,000	1,268,435,000	10,000,000		9,800,000				26,500,000																																								275,000,000
Period for which adjusted EBITDA is considered for pricing of credit facility (in months)																																							12
Cash dividend paid	81,882,000	64,969,000	68,672,000
Minimum number of common stock equity offerings necessary for proceeds to be used for debt redemption																																1
Outstanding principal balance							6,900,000	7,300,000	26,500,000	26,500,000
Outstanding letters of credit																																							16,500,000														4,000,000
Long Term Debt prior to conversion																																																															35,000,000
Principal amount of debt converted																																																									130,000,000						30,500,000
Number of term agreements																																																												1
Proceeds from sale of participation interest																																																											$ 10,000,000	$ 25,000,000
Ownership percentage																																																													81.00%	81.00%
Ownership percentage											19.00%																																																				19.00%	19.00%
Debt, weighted average interest rate (as a percent)																																																										2.80%
Interest rate percent after February 2017									5.00%