Acquisitions (Details)			12 Months Ended
	Sep. 14, 2015 USD ($)	Sep. 16, 2014 USD ($) MT	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Steel Deck Facilities Asset Purchase
Business Acquisition Information
Net working capital purchased	$ 30,000,000
Number of deck facilities purchased	2
Purchase price	$ 45,000,000
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) | MT		3.4
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
Senior Notes | Severstal Columbus LLC
Business Acquisition Information
Issuance of Senior Notes		$ 1,200,000,000