Long-Term Debt (Details) (USD $)	3 Months Ended		12 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			1 Months Ended	12 Months Ended			12 Months Ended										12 Months Ended		12 Months Ended		12 Months Ended
	Sep. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Minnesota Economic Development State Loans	Dec. 31, 2012 Minnesota Economic Development State Loans	Dec. 31, 2013 Electric Utility Development Loans	Dec. 31, 2012 Electric Utility Development Loans	Dec. 31, 2013 Term Loan maturing in 2016	Dec. 31, 2012 Term Loan maturing in 2016	Jan. 31, 2012 Term Loan maturing in 2016	Dec. 31, 2013 Term Loan maturing in 2016 Minimum	Dec. 31, 2013 Term Loan maturing in 2016 Maximum	Dec. 31, 2013 5.125% convertible senior notes, due 2014 D	Dec. 31, 2012 5.125% convertible senior notes, due 2014	Dec. 31, 2011 5.125% convertible senior notes, due 2014	Dec. 31, 2013 6 3/4% senior notes, due 2015, repaid in 2013	Apr. 30, 2013 6 3/4% senior notes, due 2015, repaid in 2013	Mar. 31, 2013 6 3/4% senior notes, due 2015, repaid in 2013	Dec. 31, 2012 6 3/4% senior notes, due 2015, repaid in 2013	Dec. 31, 2013 6 1/8% senior notes, due 2019	Dec. 31, 2012 6 1/8% senior notes, due 2019	Aug. 31, 2012 6 1/8% senior notes, due 2019	Dec. 31, 2013 6 1/8% senior notes, due 2019 Debt redemption before August 15, 2015	Dec. 31, 2013 6 1/8% senior notes, due 2019 Debt redemption on or after August 15, 2016	Dec. 31, 2013 6 1/8% senior notes, due 2019 Debt redemption on or after August 15, 2017	Dec. 31, 2013 6 1/8% senior notes, due 2019 Debt redemption on or after August 15, 2018	Dec. 31, 2013 7 5/8% senior notes, due 2020	Dec. 31, 2012 7 5/8% senior notes, due 2020	Mar. 31, 2010 7 5/8% senior notes, due 2020	Dec. 31, 2013 7 5/8% senior notes, due 2020 Debt redemption after March 15, 2015	Dec. 31, 2013 7 5/8% senior notes, due 2020 Debt redemption on or after March 15, 2016	Dec. 31, 2013 7 5/8% senior notes, due 2020 Debt redemption on or after March 15, 2017	Dec. 31, 2013 7 5/8% senior notes, due 2020 Debt redemption on or after March 15, 2018	Dec. 31, 2013 6 3/8% Senior Notes, due 2022	Dec. 31, 2012 6 3/8% Senior Notes, due 2022	Aug. 31, 2012 6 3/8% Senior Notes, due 2022	Dec. 31, 2013 6 3/8% Senior Notes, due 2022 Debt redemption before August 15, 2015	Dec. 31, 2013 6 3/8% Senior Notes, due 2022 Debt redemption on or after August 15, 2017	Dec. 31, 2013 6 3/8% Senior Notes, due 2022 Debt redemption on or after August 15, 2018	Dec. 31, 2013 6 3/8% Senior Notes, due 2022 Debt redemption on or after August 15, 2019	Dec. 31, 2013 6 3/8% Senior Notes, due 2022 Debt redemption on or after August 15, 2020	Dec. 31, 2013 5 1/4% Senior Notes due 2023	Mar. 31, 2013 5 1/4% Senior Notes due 2023	Dec. 31, 2013 5 1/4% Senior Notes due 2023 Debt redemption before April 15, 2016	Dec. 31, 2013 5 1/4% Senior Notes due 2023 Debt redemption on or after April 15, 2018	Dec. 31, 2013 5 1/4% Senior Notes due 2023 Debt redemption on or after April 15, 2019	Dec. 31, 2013 5 1/4% Senior Notes due 2023 Debt redemption on or after April 15, 2020	Dec. 31, 2013 5 1/4% Senior Notes due 2023 Debt redemption on or after April 15, 2021	Sep. 30, 2012 7 3/8% senior notes, due 2012	Dec. 31, 2012 7 3/8% senior notes, due 2012	Aug. 31, 2012 7 3/8% senior notes, due 2012	Jan. 31, 2012 7 3/8% senior notes, due 2012	Aug. 31, 2012 7 3/4% senior notes, due 2016	Dec. 31, 2012 7 3/4% senior notes, due 2016	Dec. 31, 2013 Other obligations	Dec. 31, 2012 Other obligations	Dec. 31, 2013 Senior secured revolving credit facility, due 2016	Dec. 31, 2013 Senior secured revolving credit facility, due 2016 Minimum	Dec. 31, 2013 Senior secured revolving credit facility, due 2016 Minimum LIBOR	Dec. 31, 2013 Senior secured revolving credit facility, due 2016 Minimum Prime	Dec. 31, 2013 Senior secured revolving credit facility, due 2016 Maximum	Dec. 31, 2013 Senior secured revolving credit facility, due 2016 Maximum LIBOR	Dec. 31, 2013 Senior secured revolving credit facility, due 2016 Maximum Prime	Dec. 31, 2013 Senior secured revolving credit Mesabi Nugget	Dec. 31, 2013 Term loan facilities Mesabi Nugget	Dec. 31, 2013 Mesabi Nugget Loan Participation	Dec. 31, 2013 Mesabi Nugget Loan Participation Kobe Iron Nugget, LLC (Kobe)	Dec. 31, 2013 Mesabi Nugget Loan Participation Mesabi Nugget Kobe Iron Nugget, LLC (Kobe)	Dec. 31, 2012 Revolving credit facility entered into by a controlled subsidiary subsidiary	Dec. 31, 2013 Revolving credit facility entered into by a controlled subsidiary	Dec. 31, 2013 Financing agreements entered into by a controlled subsidiary subsidiary
Long Term Debt
Total debt			$ 2,107,589,000	$ 2,202,237,000						$ 247,500,000	$ 261,250,000				$ 287,493,000	$ 287,496,000					$ 500,000,000	$ 400,000,000	$ 400,000,000						$ 350,000,000	$ 350,000,000						$ 350,000,000	$ 350,000,000							$ 400,000,000													$ 72,596,000	$ 53,491,000												$ 4,500,000
Less current maturities			341,544,000	29,631,000																																																																		901,000
Long-term debt			1,766,045,000	2,172,606,000
Stated interest rate (as a percent)						3.50%		8.10%							5.125%	5.125%	5.125%				6.75%	6.125%	6.125%	6.125%					7.625%	7.625%	7.625%					6.375%	6.375%	6.375%						5.25%	5.25%									7.375%	7.75%																		6.00%
Debt issued												275,000,000												400,000,000														350,000,000							400,000,000
Debt repurchased																			198,300,000	301,700,000																																	62,200,000	279,700,000	410,500,000
Expenses recorded related to tender offer and early payoff of Senior Notes related to tender premiums, unamortized debt issuance cost write-off, and tender expenses																		2,600,000
Debt extinguished																																																			358,100,000				89,500,000
Repayments of debt			517,978,000	1,258,842,000	7,740,000																																															700,000,000				500,000,000
Maximum borrowing capacity of credit facility																																																											1,100,000,000							80,000,000	79,500,000					34,000,000
Outstanding principal balance						27,200,000	28,800,000	6,000,000	6,400,000																																																														10,000,000	20,100,000	11,200,000
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%
Unpaid principal balance payment			183,132,000							158,000,000
Effective interest rate ( as a percent)										1.80%										6.75%																																																				3.25%
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%
Availability on the senior secured revolver																																																											1,100,000,000
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%
Outstanding letters of credit and other obligations which reduce availability																																																											13,800,000
Borrowings outstanding																																																											0
Consolidated interest coverage ratio																																																											5.30	2.50
Leverage ratio																																																											2.60				5.00
Threshold debt leverage ratio upon exceeding which payment restrictions occur																																																															3.50
Convertible debt rate, basis for conversion															0.058342
Convertible debt, shares issued															16,772,830
Threshold amount to pay cash dividends or distributions to holders of common stock (in dollars per share)															$ 0.075
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															20
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															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)															$ 17.14
Redemption price of debt instrument (as a percent)															100.00%
Maximum redemption of debt instrument (as a percent)																									35.00%														35.00%							35.00%
Redemption price of debt instrument (as a percent)																									106.125%	103.063%	101.531%	100.00%				103.813%	102.542%	101.271%	100.00%				106.375%	103.188%	102.125%	101.063%	100.00%			105.25%	102.625%	101.75%	100.875%	100.00%
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
Proceeds from sale of participation interest																																																																					4,500,000
Debt, weighted average interest rate (as a percent)																																																																				2.70%
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
Number of company's controlled subsidiaries that entered into an agreement																																																																							1		1