Condensed Consolidating Financial Statements - Balance Sheet (Details) - USD ($) $ in Millions	1 Months Ended
	Apr. 30, 2013	Nov. 30, 2006	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Condensed Unaudited Consolidating Balance Sheet
Ownership interest in domestic subsidiaries (as a percent)			100.00%
Assets
Cash and cash equivalents			$ 120.5	$ 104.3	$ 101.6	$ 106.2
Accounts receivable, net			1,009.0	916.6
Inventories			1,525.7	1,436.0
Income taxes receivable			29.3	36.5
Other current assets			55.4	60.8
Total current assets			2,739.9	2,554.2
Property, plant and equipment, net			1,671.9	1,635.5
Goodwill			1,824.6	1,724.8
Intangible assets, net			1,205.0	1,125.4
Other assets			78.1	81.7
Total assets			7,519.5	7,121.6
Liabilities & Equity
Accounts payable			350.4	247.0
Accrued compensation and retirement costs			91.0	118.7
Other current liabilities			136.2	123.2
Current maturities of long-term debt and short-term borrowings			493.8	500.8
Total current liabilities			1,071.4	989.7
Long-term debt			1,640.3	1,427.9
Other long-term liabilities			767.4	761.3
Total Reliance stockholders' equity			4,012.0	3,914.1
Noncontrolling interests			28.4	28.6
Total equity			4,040.4	3,942.7
Total liabilities and equity			7,519.5	7,121.6
Senior Unsecured Notes
Condensed Unaudited Consolidating Balance Sheet
Aggregate principal amount	$ 1,100.0	$ 1,100.0
Ownership interest in domestic subsidiaries (as a percent)	100.00%	100.00%
Reportable legal entities | Parent
Assets
Cash and cash equivalents			17.1	10.7	26.6	41.9
Accounts receivable, net			75.2	66.5
Inventories			61.7	56.0
Income taxes receivable			47.7	38.2
Other current assets			30.1	36.4
Total current assets			231.8	207.8
Investments in subsidiaries			5,172.2	5,046.2
Property, plant and equipment, net			115.8	115.5
Goodwill			23.8	23.8
Intangible assets, net			13.5	14.0
Intercompany receivables			787.3	614.2
Other assets			24.5	24.6
Total assets			6,368.9	6,046.1
Liabilities & Equity
Accounts payable			33.7	25.3
Accrued compensation and retirement costs			15.4	29.5
Other current liabilities			67.2	54.0
Current maturities of long-term debt and short-term borrowings			400.0	400.0
Total current liabilities			516.3	508.8
Long-term debt			1,629.7	1,417.3
Other long-term liabilities			210.9	205.9
Total Reliance stockholders' equity			4,012.0	3,914.1
Total equity			4,012.0	3,914.1
Total liabilities and equity			6,368.9	6,046.1
Reportable legal entities | Guarantor Subsidiaries
Assets
Cash and cash equivalents			(4.8)	(1.0)	(3.5)	(8.3)
Accounts receivable, net			775.8	701.8
Inventories			1,242.0	1,145.1
Other current assets			13.1	15.1
Total current assets			2,026.1	1,861.0
Investments in subsidiaries			340.0	328.0
Property, plant and equipment, net			1,345.5	1,311.3
Goodwill			1,667.1	1,571.1
Intangible assets, net			1,055.1	977.5
Intercompany receivables			47.5	45.4
Other assets			47.6	50.9
Total assets			6,528.9	6,145.2
Liabilities & Equity
Accounts payable			252.0	160.0
Accrued compensation and retirement costs			69.6	79.1
Other current liabilities			34.2	15.3
Total current liabilities			355.8	254.4
Long-term debt			5.7	5.7
Intercompany borrowings			712.6	547.6
Other long-term liabilities			505.2	505.0
Total Reliance stockholders' equity			4,942.3	4,824.5
Noncontrolling interests			7.3	8.0
Total equity			4,949.6	4,832.5
Total liabilities and equity			6,528.9	6,145.2
Reportable legal entities | Non-Guarantor Subsidiaries
Assets
Cash and cash equivalents			108.2	94.6	$ 78.5	$ 72.6
Accounts receivable, net			162.2	152.3
Inventories			222.0	234.9
Income taxes receivable				(1.7)
Other current assets			13.5	12.6
Total current assets			505.9	492.7
Property, plant and equipment, net			210.6	208.7
Goodwill			133.7	129.9
Intangible assets, net			136.4	133.9
Intercompany receivables			32.0	30.5
Other assets			6.0	6.2
Total assets			1,024.6	1,001.9
Liabilities & Equity
Accounts payable			68.9	65.7
Accrued compensation and retirement costs			6.0	10.1
Other current liabilities			54.5	57.2
Current maturities of long-term debt and short-term borrowings			93.8	100.8
Total current liabilities			223.2	233.8
Long-term debt			4.9	4.9
Intercompany borrowings			154.2	142.5
Other long-term liabilities			51.3	50.4
Total Reliance stockholders' equity			569.9	549.7
Noncontrolling interests			21.1	20.6
Total equity			591.0	570.3
Total liabilities and equity			1,024.6	1,001.9
Consolidating Adjustments
Assets
Accounts receivable, net			(4.2)	(4.0)
Income taxes receivable			(18.4)
Other current assets			(1.3)	(3.3)
Total current assets			(23.9)	(7.3)
Investments in subsidiaries			(5,512.2)	(5,374.2)
Intercompany receivables			(866.8)	(690.1)
Total assets			(6,402.9)	(6,071.6)
Liabilities & Equity
Accounts payable			(4.2)	(4.0)
Other current liabilities			(19.7)	(3.3)
Total current liabilities			(23.9)	(7.3)
Intercompany borrowings			(866.8)	(690.1)
Total Reliance stockholders' equity			(5,512.2)	(5,374.2)
Total equity			(5,512.2)	(5,374.2)
Total liabilities and equity			$ (6,402.9)	$ (6,071.6)