Long-Term Debt (Details) (USD $)	3 Months Ended		12 Months Ended				24 Months Ended					1 Months Ended		12 Months Ended		12 Months Ended				12 Months Ended		0 Months Ended		12 Months Ended		0 Months Ended		12 Months Ended				1 Months Ended			12 Months Ended					0 Months Ended		12 Months Ended							12 Months Ended										1 Months Ended						12 Months Ended		12 Months Ended				12 Months Ended			12 Months Ended
	Sep. 30, 2012	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Minnesota Economic Development State Loans	Dec. 31, 2010 Minnesota Economic Development State Loans	Dec. 31, 2011 Minnesota Economic Development State Loans	Dec. 31, 2012 Electric Utility Development Loans	Dec. 31, 2011 Electric Utility Development Loans	Dec. 31, 2012 Mesabi Nugget Kobe Iron Nugget, LLC (Kobe)	Aug. 31, 2012 7 3/8% senior notes, due 2012	Jan. 31, 2012 7 3/8% senior notes, due 2012	Dec. 31, 2012 7 3/8% senior notes, due 2012	Dec. 31, 2011 7 3/8% senior notes, due 2012	Dec. 31, 2012 5.125% convertible senior notes, due 2014	Dec. 31, 2011 5.125% convertible senior notes, due 2014	Dec. 31, 2012 6 3/4% senior notes, due 2015	Dec. 31, 2011 6 3/4% senior notes, due 2015	Dec. 31, 2012 6 3/4% senior notes, due 2015 Debt redemption on or after April 1, 2012	Dec. 31, 2012 6 3/4% senior notes, due 2015 Debt redemption on or after April 1, 2013	Aug. 31, 2012 7 3/4% senior notes, due 2016	Aug. 16, 2012 7 3/4% senior notes, due 2016	Dec. 31, 2012 7 3/4% senior notes, due 2016	Dec. 31, 2011 7 3/4% senior notes, due 2016	Aug. 16, 2012 6 1/8% senior notes, due 2019	Dec. 31, 2012 6 1/8% senior notes, due 2019	Dec. 31, 2012 6 1/8% senior notes, due 2019 Debt redemption before August 15, 2015	Dec. 31, 2012 6 1/8% senior notes, due 2019 Debt redemption on or after August 15, 2016	Dec. 31, 2012 6 1/8% senior notes, due 2019 Debt redemption on or after August 15, 2017	Dec. 31, 2012 6 1/8% senior notes, due 2019 Debt redemption on or after August 15, 2018	Mar. 31, 2010 7 5/8% senior notes, due 2020	Dec. 31, 2012 7 5/8% senior notes, due 2020	Dec. 31, 2011 7 5/8% senior notes, due 2020	Dec. 31, 2012 7 5/8% senior notes, due 2020 Debt redemption prior to March 15, 2013	Dec. 31, 2012 7 5/8% senior notes, due 2020 Debt redemption after March 15, 2015	Dec. 31, 2012 7 5/8% senior notes, due 2020 Debt redemption on or after March 15, 2016	Dec. 31, 2012 7 5/8% senior notes, due 2020 Debt redemption on or after March 15, 2017	Dec. 31, 2012 7 5/8% senior notes, due 2020 Debt redemption on or after March 15, 2018	Aug. 16, 2012 6 3/8% Senior Notes, due 2022	Dec. 31, 2012 6 3/8% Senior Notes, due 2022	Dec. 31, 2012 6 3/8% Senior Notes, due 2022 Debt redemption before August 15, 2015	Dec. 31, 2012 6 3/8% Senior Notes, due 2022 Debt redemption on or after August 15, 2017	Dec. 31, 2012 6 3/8% Senior Notes, due 2022 Debt redemption on or after August 15, 2018	Dec. 31, 2012 6 3/8% Senior Notes, due 2022 Debt redemption on or after August 15, 2019	Dec. 31, 2012 6 3/8% Senior Notes, due 2022 Debt redemption on or after August 15, 2020	Dec. 31, 2012 Other secured obligations	Dec. 31, 2011 Other secured obligations	Dec. 31, 2012 Senior secured revolving credit facility, due 2016	Dec. 31, 2012 Senior secured revolving credit facility, due 2016 Scenario actual	Dec. 31, 2012 Senior secured revolving credit facility, due 2016 Minimum	Dec. 31, 2012 Senior secured revolving credit facility, due 2016 Minimum Scenario forecasted	Dec. 31, 2012 Senior secured revolving credit facility, due 2016 Minimum LIBOR	Dec. 31, 2012 Senior secured revolving credit facility, due 2016 Minimum Prime	Dec. 31, 2012 Senior secured revolving credit facility, due 2016 Maximum	Dec. 31, 2012 Senior secured revolving credit facility, due 2016 Maximum Scenario forecasted	Dec. 31, 2012 Senior secured revolving credit facility, due 2016 Maximum LIBOR	Dec. 31, 2012 Senior secured revolving credit facility, due 2016 Maximum Prime	Jan. 31, 2012 Term Loan maturing in 2016	Dec. 31, 2012 Term Loan maturing in 2016	Dec. 31, 2012 Term Loan maturing in 2016 Minimum	Dec. 31, 2012 Term Loan maturing in 2016 Maximum	Dec. 31, 2012 Senior secured revolving credit facility after amendment Mesabi Nugget	Dec. 31, 2012 Term loan facilities Mesabi Nugget	Dec. 31, 2010 Mesabi Nugget Loan Participation	Dec. 31, 2012 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, 2012 Mesabi Nugget Loan Participation Mesabi Nugget Kobe Iron Nugget, LLC (Kobe)	Dec. 31, 2012 Term borrowings entered into by a controlled subsidiary	Dec. 31, 2012 Revolving credit facility entered into by a controlled subsidiary
Long Term Debt
Total debt			$ 2,202,237,000	$ 2,380,100,000											$ 700,000,000	$ 287,496,000	$ 287,500,000	$ 500,000,000	$ 500,000,000						$ 500,000,000		$ 400,000,000						$ 350,000,000	$ 350,000,000							$ 350,000,000						$ 53,491,000	$ 42,600,000												$ 261,250,000												$ 4,500,000
Less current maturities			29,631,000	444,078,000																																																																				901,000
Long-term debt			2,172,606,000	1,936,022,000
Stated interest rate (as a percent)						3.50%			8.10%						7.375%	5.125%	5.125%	6.75%	6.75%						7.75%		6.125%						7.625%	7.625%							6.375%
Debt issued																										400,000,000						350,000,000								350,000,000																			275,000,000
Maximum availability of credit facility after April 2013																																																																										34,000,000
Maximum borrowing capacity of credit facility																																																	1,100,000,000														85,000,000	240,000,000
Outstanding principal balance						28,800,000		26,500,000	6,400,000	6,900,000
Additional amount by which facility size can be increased																																																	125,000,000
Quarterly principal payment as a percent of the original principal amount																																																													1.25%	3.75%
Effective interest rate ( as a percent)																																																												2.10%													3.375%
Maximum redemption of debt instrument (as a percent)																												35.00%							35.00%							35.00%
Unpaid principal balance payment			3,037,000																																																									158,000,000
Gross amount of debt extinguished														358,100,000
Redemption of debt												62,200,000	279,700,000	700,000,000								89,500,000	410,500,000	500,000,000
Overall reduction of outstanding long-term debt due to refinancing activity			175,000,000
Expenses recorded for tender and call premiums, loss on early extinguishment of debt, unamortized debt issuance cost write-off, and tender expenses	26,400,000	13,900,000	40,300,000
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																																																	14,000,000
Consolidated interest coverage ratio																																																		4.05		2.50
Leverage ratio																																																		2.93						5.00
Threshold debt leverage ratio upon exceeding which payment restrictions occur																																																								3.50
Convertible debt, shares, basis for conversion																0.057845
Convertible debt, shares issued																16,630,206
Threshold amount to pay cash dividends or distributions to holders of common stock (in dollars per share)																$ 0.075
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 days
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																30 days
Minimum sales price as percentage of conversion price considered for redemption of notes																130.00%
Conversion price per share (in dollars per share)																$ 17.29
Redemption price of debt instrument (as a percent)																100.00%				101.688%	100.00%							106.125%	103.063%	101.531%	100.00%				107.625%	103.813%	102.542%	101.271%	100.00%			106.375%	103.188%	102.125%	101.063%	100.00%
Proceeds from debt			1,049,969,000	10,103,000	556,553,000		26,500,000
Principal amount of debt converted																																																																	130,000,000						30,500,000
Long Term Debt prior to conversion																																																																							35,000,000
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)																																																																		3.00%
Interest rate percent after February 2017						5.00%
Period for which adjusted EBITDA is considered for pricing of credit facility																																																	12 months
Outstanding letters of credit									4,000,000
Maximum availability of credit facility																																																																									$ 10,000,000	$ 24,000,000