Summary of Significant Accounting Policies (Financial Statement Line Items in our Consolidated Statements of Operations and Consolidated Statements of Cash Flows) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Jan. 01, 2019
Income Statement [Abstract]
Cost of goods sold									$ 4,034.3	$ 3,376.1	$ 3,728.5
Other (income) expense, net									(17.9)	(22.4)	1.4
Earnings before interest and income taxes									596.0	407.5	486.8
Income taxes									119.5	74.8	89.4
Net earnings	$ 105.6	$ 97.2	$ 112.3	$ 87.5	$ 108.0	$ 107.1	$ (6.1)	$ 44.1	$ 402.6	$ 253.1	$ 314.1
Net earnings per share attributable to Leggett & Platt, Inc. common shareholders:
Basic (in dollars per share)	$ 0.77	$ 0.71	$ 0.83	$ 0.64	$ 0.79	$ 0.79	$ (0.05)	$ 0.33	$ 2.95	$ 1.86	$ 2.33
Diluted (in dollars per share)	$ 0.77	$ 0.71	$ 0.82	$ 0.64	$ 0.79	$ 0.79	$ (0.05)	$ 0.33	$ 2.94	$ 1.86	$ 2.32
Statement of Financial Position [Abstract]
Inventories	$ 993.2				$ 691.5				$ 993.2	$ 691.5
Deferred income taxes	217.4				205.4				217.4	205.4
Retained earnings	2,973.0				2,797.2				2,973.0	2,797.2		$ 49.2
Statement of Cash Flows [Abstract]
Net earnings	105.6	$ 97.2	$ 112.3	$ 87.5	108.0	$ 107.1	$ (6.1)	$ 44.1	402.6	253.1	$ 314.1
Writedown of inventories									13.7	13.6	15.1
Net gain from sales of assets and businesses									(29.4)	0.0	(5.0)
Deferred income tax (benefit) expense									(8.5)	(20.9)	0.8
Inventories									(305.0)	(31.9)	73.8
Change In Inventory Valuation
Income Statement [Abstract]
Cost of goods sold										3,376.1	3,728.5
Other (income) expense, net										(22.4)
Earnings before interest and income taxes										407.5	486.8
Income taxes										74.8	89.4
Net earnings										$ 253.1	$ 314.1
Net earnings per share attributable to Leggett & Platt, Inc. common shareholders:
Basic (in dollars per share)										$ 1.86	$ 2.33
Diluted (in dollars per share)										$ 1.86	$ 2.32
Statement of Financial Position [Abstract]
Inventories					691.5					$ 691.5
Deferred income taxes					205.4					205.4
Retained earnings					2,797.2					2,797.2
Statement of Cash Flows [Abstract]
Net earnings										253.1	$ 314.1
Writedown of inventories										13.6	15.1
Net gain from sales of assets and businesses										0.0
Deferred income tax (benefit) expense										(20.9)	0.8
Inventories										(31.9)	73.8
As Adjusted | Change In Inventory Valuation
Income Statement [Abstract]
Cost of goods sold									4,101.2	3,385.7	3,701.9
Other (income) expense, net										(25.0)
Earnings before interest and income taxes									529.1	400.5	513.4
Income taxes									103.0	73.2	96.2
Net earnings									$ 352.2	$ 247.7	$ 333.9
Net earnings per share attributable to Leggett & Platt, Inc. common shareholders:
Basic (in dollars per share)									$ 2.58	$ 1.82	$ 2.48
Diluted (in dollars per share)									$ 2.57	$ 1.82	$ 2.47
Statement of Financial Position [Abstract]
Inventories	880.3				645.5				$ 880.3	$ 645.5
Deferred income taxes	189.7				194.2				189.7	194.2
Retained earnings	2,887.8				2,762.4				2,887.8	2,762.4
Statement of Cash Flows [Abstract]
Net earnings									352.2	247.7	$ 333.9
Writedown of inventories									11.5	10.9	9.0
Net gain from sales of assets and businesses										(2.5)
Deferred income tax (benefit) expense									(25.0)	(22.5)	7.6
Inventories									(235.9)	(19.7)	53.3
Effect of Change | Change In Inventory Valuation
Income Statement [Abstract]
Cost of goods sold									66.9	(9.6)	26.6
Other (income) expense, net										2.6
Earnings before interest and income taxes									(66.9)	7.0	(26.6)
Income taxes									(16.5)	1.6	(6.8)
Net earnings									$ (50.4)	$ 5.4	$ (19.8)
Net earnings per share attributable to Leggett & Platt, Inc. common shareholders:
Basic (in dollars per share)									$ (0.37)	$ 0.04	$ (0.15)
Diluted (in dollars per share)									$ (0.37)	$ 0.04	$ (0.15)
Statement of Financial Position [Abstract]
Inventories	(112.9)				46.0				$ (112.9)	$ 46.0
Deferred income taxes	(27.7)				11.2				(27.7)	11.2
Retained earnings	$ (85.2)				$ 34.8				(85.2)	34.8
Statement of Cash Flows [Abstract]
Net earnings									(50.4)	5.4	$ (19.8)
Writedown of inventories									(2.2)	2.7	6.1
Net gain from sales of assets and businesses										2.5
Deferred income tax (benefit) expense									(16.5)	1.6	(6.8)
Inventories									$ 69.1	$ (12.2)	$ 20.5