Condensed Consolidating Financial Statements (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Consolidating financial statements, captions
Net sales	$ 1,889.0	$ 2,055.3	$ 2,209.7	$ 2,288.3	$ 2,033.9	$ 2,138.6	$ 2,049.5	$ 1,912.7	$ 8,442.3	$ 8,134.7	$ 6,312.8
Costs and expenses:
Cost of sales (exclusive of depreciation and amortization shown below)	1,364.6	1,520.0	1,640.3	1,710.5	1,558.9	1,644.7	1,538.7	1,406.4	6,235.4	6,148.7	4,727.9
Warehouse, delivery, selling, general and administrative									1,396.2	1,280.1	1,103.6
Depreciation and amortization									151.5	133.1	120.6
Total costs and expenses									7,783.1	7,561.9	5,952.1
Operating income									659.2	572.8	360.7
Other income (expense):
Interest									(58.4)	(59.8)	(61.2)
Other income net									8.6	(1.4)	(3.0)
Income before equity in earnings of subsidiaries and income taxes									609.4	511.6	296.5
Income before income taxes									609.4	511.6	296.5
Income tax (benefit) provision									201.1	162.4	98.6
Net income	80.8	99.4	110.2	117.9	69.1	86.3	100.2	93.6	408.3	349.2	197.9
Less: Net income attributable to noncontrolling interests									4.8	5.4	3.5
Net income attributable to Reliance	80.4	98.1	108.8	116.2	67.9	84.9	98.7	92.3	403.5	343.8	194.4
Parent
Consolidating financial statements, captions
Net sales									726.3	706.8	593.7
Costs and expenses:
Cost of sales (exclusive of depreciation and amortization shown below)									534.9	535.3	442.4
Warehouse, delivery, selling, general and administrative									188.1	72.0	116.3
Depreciation and amortization									14.3	14.3	13.0
Total costs and expenses									737.3	621.6	571.7
Operating income									(11.0)	85.2	22.0
Other income (expense):
Interest									(57.7)	(58.9)	(62.4)
Other income net									73.9	93.4	51.0
Income before equity in earnings of subsidiaries and income taxes									5.2	119.7	10.6
Equity in earnings of subsidiaries									379.2	204.5	144.3
Income before income taxes									384.4	324.2	154.9
Income tax (benefit) provision									(19.1)	(19.6)	(39.5)
Net income									403.5	343.8	194.4
Net income attributable to Reliance									403.5	343.8	194.4
Guarantor Subsidiaries
Consolidating financial statements, captions
Net sales									7,298.4	7,103.0	5,550.6
Costs and expenses:
Cost of sales (exclusive of depreciation and amortization shown below)									5,463.2	5,461.0	4,226.0
Warehouse, delivery, selling, general and administrative									1,164.5	1,187.8	978.7
Depreciation and amortization									124.7	109.8	101.3
Total costs and expenses									6,752.4	6,758.6	5,306.0
Operating income									546.0	344.4	244.6
Other income (expense):
Interest									(14.4)	(27.6)	(33.8)
Other income net									3.4	2.6	44.8
Income before equity in earnings of subsidiaries and income taxes									535.0	319.4	255.6
Equity in earnings of subsidiaries									28.9	32.1	10.7
Income before income taxes									563.9	351.5	266.3
Income tax (benefit) provision									206.0	166.7	130.6
Net income									357.9	184.8	135.7
Less: Net income attributable to noncontrolling interests									4.6	4.6	3.0
Net income attributable to Reliance									353.3	180.2	132.7
Non-Guarantor Subsidiaries
Consolidating financial statements, captions
Net sales									640.2	547.8	338.2
Costs and expenses:
Cost of sales (exclusive of depreciation and amortization shown below)									460.0	375.4	229.4
Warehouse, delivery, selling, general and administrative									96.4	86.8	71.5
Depreciation and amortization									12.5	9.0	6.3
Total costs and expenses									568.9	471.2	307.2
Operating income									71.3	76.6	31.0
Other income (expense):
Interest									(2.2)	(2.1)	(0.8)
Other income net									0.1	(2.0)	0.1
Income before equity in earnings of subsidiaries and income taxes									69.2	72.5	30.3
Income before income taxes									69.2	72.5	30.3
Income tax (benefit) provision									14.2	15.3	7.5
Net income									55.0	57.2	22.8
Less: Net income attributable to noncontrolling interests									0.2	0.8	0.5
Net income attributable to Reliance									54.8	56.4	22.3
Eliminations
Consolidating financial statements, captions
Net sales									(222.6)	(222.9)	(169.7)
Costs and expenses:
Cost of sales (exclusive of depreciation and amortization shown below)									(222.7)	(223.0)	(169.9)
Warehouse, delivery, selling, general and administrative									(52.8)	(66.5)	(62.9)
Total costs and expenses									(275.5)	(289.5)	(232.8)
Operating income									52.9	66.6	63.1
Other income (expense):
Interest									15.9	28.8	35.8
Other income net									(68.8)	(95.4)	(98.9)
Equity in earnings of subsidiaries									(408.1)	(236.6)	(155.0)
Income before income taxes									(408.1)	(236.6)	(155.0)
Net income									(408.1)	(236.6)	(155.0)
Net income attributable to Reliance									$ (408.1)	$ (236.6)	$ (155.0)