Acquisition (Details) (USD $)	3 Months Ended	12 Months Ended			0 Months Ended	3 Months Ended
	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Sep. 16, 2014 MT	Dec. 31, 2014
Business Acquisition Information
Acquisition and related costs	$ 25,000,000
Net assets acquired
Goodwill		745,158,000	731,996,000			745,158,000
Amortization of intangible assets
Amortization of intangible assets		27,551,000	31,770,000	35,553,000
Estimated amortization expense
2015		24,180,000				24,180,000
2016		21,939,000				21,939,000
2017		19,157,000				19,157,000
2018		16,623,000				16,623,000
2019		15,084,000				15,084,000
Thereafter		83,886,000				83,886,000
Total		180,869,000				180,869,000
Severstal Columbus LLC
Business Acquisition Information
Business acquisition percentage					100.00%
Purchase price					1,625,000,000
Working capital adjustments					44,400,000
Additional production capacity due to acquisition (in million tons)					3.4
Revenue since acquisition						638,300,000
Pre-tax income since acquisition						29,800,000
Purchase accounting related expenses						26,400,000
Acquisition and related costs		25,200,000
Net assets acquired
Current assets, net of cash acquired					553,255,000
Property, plant & equipment					1,279,714,000
Intangible assets					15,000,000
Goodwill					19,682,000
Other assets					3,875,000
Total assets acquired					1,871,526,000
Liabilities assumed					202,077,000
Net assets acquired					1,669,449,000
Amortization of intangible assets
Intangible assets					15,000,000
Amortization of intangible assets		4,400,000
Estimated amortization expense
2015		790,000				790,000
2016		1,038,000				1,038,000
2017		964,000				964,000
2018		859,000				859,000
2019		753,000				753,000
Thereafter		6,196,000				6,196,000
Total		10,600,000				10,600,000
Unaudited pro forma results
Net sales		10,355,774,000	9,193,344,000
Net Income attributable to Steel Dynamics, Inc.		264,779,000	155,357,000
Pro forma step-up inventory expense			19,700,000
Pro-forma backlogs			4,000,000
Pro forma acquisition costs			25,200,000
Severstal Columbus LLC | Customer and scrap generator relationships
Amortization of intangible assets
Intangible assets					11,000,000
Expected life					15 years
Amortization of intangible assets		400,000
Severstal Columbus LLC | Backlogs
Amortization of intangible assets
Intangible assets					4,000,000
Expected life					2 months
Amortization of intangible assets		4,000,000
Senior Notes | Severstal Columbus LLC
Business Acquisition Information
Issuance of Senior Notes					$ 1,200,000,000