Condensed Consolidating Financial Statements - Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Condensed Unaudited Consolidating Statement of Comprehensive Income
Net sales	$ 2,026.2	$ 2,286.2	$ 2,423.7	$ 2,614.4	$ 2,576.7	$ 2,705.1	$ 2,616.8	$ 2,553.0	$ 9,350.5	$ 10,451.6	$ 9,223.8
Costs and expenses:
Cost of sales (exclusive of depreciation and amortization shown below)	1,444.2	1,647.9	1,767.8	1,943.7	1,954.4	2,026.9	1,943.5	1,905.8	6,803.6	7,830.6	6,826.2
Warehouse, delivery, selling, general and administrative									1,728.5	1,789.8	1,638.4
Depreciation and amortization									218.5	213.8	192.4
Impairment of long-lived assets									53.3		14.9
Total costs and expenses									8,803.9	9,834.2	8,671.9
Operating (loss) income									546.6	617.4	551.9
Other income (expense):
Interest									(84.3)	(81.9)	(77.5)
Other (expense) income, net									(3.6)	10.8	3.9
(Loss) income before equity in earnings of subsidiaries and income taxes									458.7	546.3	478.3
Income before income taxes									458.7	546.3	478.3
Income tax (benefit) provision									142.5	170.0	153.6
Net income	68.9	52.8	91.6	102.9	93.7	96.9	97.8	87.9	316.2	376.3	324.7
Less: Net income attributable to noncontrolling interests									4.7	4.8	3.1
Net income attributable to Reliance	$ 68.6	$ 51.4	$ 90.2	$ 101.3	$ 92.3	$ 95.5	$ 96.5	$ 87.2	311.5	371.5	321.6
Comprehensive income attributable to Reliance									260.7	329.3	316.4
Reportable legal entities | Parent
Condensed Unaudited Consolidating Statement of Comprehensive Income
Net sales									710.9	750.2	704.5
Costs and expenses:
Cost of sales (exclusive of depreciation and amortization shown below)									503.1	557.0	509.7
Warehouse, delivery, selling, general and administrative									168.1	157.3	200.7
Depreciation and amortization									16.8	19.8	17.5
Total costs and expenses									688.0	734.1	727.9
Operating (loss) income									22.9	16.1	(23.4)
Other income (expense):
Interest									(80.3)	(78.1)	(74.7)
Other (expense) income, net									17.2	67.7	75.7
(Loss) income before equity in earnings of subsidiaries and income taxes									(40.2)	5.7	(22.4)
Equity in earnings of subsidiaries									297.8	325.1	300.3
Income before income taxes									257.6	330.8	277.9
Income tax (benefit) provision									(53.9)	(40.7)	(43.7)
Net income									311.5	371.5	321.6
Net income attributable to Reliance									311.5	371.5	321.6
Comprehensive income attributable to Reliance									260.7	367.5	318.9
Reportable legal entities | Guarantor Subsidiaries
Condensed Unaudited Consolidating Statement of Comprehensive Income
Net sales									7,978.3	9,110.2	8,021.6
Costs and expenses:
Cost of sales (exclusive of depreciation and amortization shown below)									5,833.9	6,885.4	5,999.2
Warehouse, delivery, selling, general and administrative									1,435.8	1,545.9	1,385.5
Depreciation and amortization									179.1	174.0	158.6
Impairment of long-lived assets									51.8		14.9
Total costs and expenses									7,500.6	8,605.3	7,558.2
Operating (loss) income									477.7	504.9	463.4
Other income (expense):
Interest									(19.2)	(28.3)	(19.9)
Other (expense) income, net									(0.7)	(2.7)	5.5
(Loss) income before equity in earnings of subsidiaries and income taxes									457.8	473.9	449.0
Equity in earnings of subsidiaries									13.1	21.2	12.4
Income before income taxes									470.9	495.1	461.4
Income tax (benefit) provision									182.5	190.1	181.0
Net income									288.4	305.0	280.4
Less: Net income attributable to noncontrolling interests									4.7	4.4	2.8
Net income attributable to Reliance									283.7	300.6	277.6
Comprehensive income attributable to Reliance									250.1	280.4	293.5
Reportable legal entities | Non-Guarantor Subsidiaries
Condensed Unaudited Consolidating Statement of Comprehensive Income
Net sales									864.6	825.9	710.1
Costs and expenses:
Cost of sales (exclusive of depreciation and amortization shown below)									670.0	623.0	529.7
Warehouse, delivery, selling, general and administrative									140.0	140.9	117.8
Depreciation and amortization									22.6	20.0	16.3
Impairment of long-lived assets									1.5
Total costs and expenses									834.1	783.9	663.8
Operating (loss) income									30.5	42.0	46.3
Other income (expense):
Interest									(6.7)	(5.7)	(4.2)
Other (expense) income, net									17.3	30.4	9.6
(Loss) income before equity in earnings of subsidiaries and income taxes									41.1	66.7	51.7
Income before income taxes									41.1	66.7	51.7
Income tax (benefit) provision									13.9	20.6	16.3
Net income									27.2	46.1	35.4
Less: Net income attributable to noncontrolling interests										0.4	0.3
Net income attributable to Reliance									27.2	45.7	35.1
Comprehensive income attributable to Reliance									(18.9)	27.7	16.7
Consolidating Adjustments
Condensed Unaudited Consolidating Statement of Comprehensive Income
Net sales									(203.3)	(234.7)	(212.4)
Costs and expenses:
Cost of sales (exclusive of depreciation and amortization shown below)									(203.4)	(234.8)	(212.4)
Warehouse, delivery, selling, general and administrative									(15.4)	(54.3)	(65.6)
Total costs and expenses									(218.8)	(289.1)	(278.0)
Operating (loss) income									15.5	54.4	65.6
Other income (expense):
Interest									21.9	30.2	21.3
Other (expense) income, net									(37.4)	(84.6)	(86.9)
Equity in earnings of subsidiaries									(310.9)	(346.3)	(312.7)
Income before income taxes									(310.9)	(346.3)	(312.7)
Net income									(310.9)	(346.3)	(312.7)
Net income attributable to Reliance									(310.9)	(346.3)	(312.7)
Comprehensive income attributable to Reliance									$ (231.2)	$ (346.3)	$ (312.7)