Supplemental Guarantor Information - Condensed Consolidating Balance Sheet (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Current assets
Cash and cash equivalents	$ 64.7		$ 86.3		$ 81.3	$ 81.3	$ 216.9
Accounts and notes receivable, net	402.4	$ 485.8	483.1
Inventories	387.3	355.3	356.9
Other current assets	135.2	116.1	114.5
Current assets held for sale	0.0	711.8	708.0
Total current assets	989.6		1,748.8
Property, plant and equipment, net	274.2		279.8
Other assets
Investments in subsidiaries	0.0		0.0
Goodwill	2,097.0		2,112.8
Intangibles, net	289.4		321.8
Long-term intercompany debt			0.0
Other non-current assets	159.3	136.0	180.9
Non-current assets held for sale	0.0	3,788.0	3,989.6
Total other assets	2,545.7		6,605.1
Total assets	3,809.5		8,633.7
Current liabilities
Accounts payable	261.3		321.5
Employee compensation and benefits	85.0		115.8
Other current liabilities	361.1	404.0	401.3
Current liabilities held for sale	0.0		360.8
Total current liabilities	707.4		1,199.4
Other liabilities
Long-term debt	798.8		1,440.7
Pension and other post-retirement compensation and benefits	109.8		96.4
Deferred tax liabilities	106.3	105.0	108.6
Other non-current liabilities	207.0		213.8
Non-current liabilities held for sale	0.0	$ 510.4	537.0
Total liabilities	1,929.3		3,595.9
Equity
Total equity	1,880.2		5,037.8	$ 5,008.6			4,254.4
Total liabilities and equity	3,809.5		8,633.7
Parent Company Guarantor
Current assets
Cash and cash equivalents	0.1		0.0		0.1		0.0
Accounts and notes receivable, net	0.0		0.0
Inventories	0.0		0.0
Other current assets	14.3		10.8
Current assets held for sale			0.0
Total current assets	14.4		10.8
Property, plant and equipment, net	0.0		0.0
Other assets
Investments in subsidiaries	1,908.9		5,205.1
Goodwill	0.0		0.0
Intangibles, net	0.0		0.0
Long-term intercompany debt			0.0
Other non-current assets	23.4		2.2
Non-current assets held for sale			0.0
Total other assets	1,932.3		5,207.3
Total assets	1,946.7		5,218.1
Current liabilities
Accounts payable	2.0		1.4
Employee compensation and benefits	0.5		0.4
Other current liabilities	42.1		99.6
Current liabilities held for sale			0.0
Total current liabilities	44.6		101.4
Other liabilities
Long-term debt	0.0		48.4
Pension and other post-retirement compensation and benefits	0.0		0.0
Deferred tax liabilities	0.0		0.0
Other non-current liabilities	21.9		30.5
Non-current liabilities held for sale			0.0
Total liabilities	66.5		180.3
Equity
Total equity	1,880.2		5,037.8
Total liabilities and equity	1,946.7		5,218.1
Guarantor Subsidiaries
Current assets
Cash and cash equivalents	0.0		0.0		0.0		0.0
Accounts and notes receivable, net	0.0		0.0
Inventories	0.0		0.0
Other current assets	0.0		1.8
Current assets held for sale			0.0
Total current assets	0.0		1.8
Property, plant and equipment, net	0.0		0.0
Other assets
Investments in subsidiaries	1,840.2		5,109.6
Goodwill	0.0		0.0
Intangibles, net	0.0		0.0
Long-term intercompany debt			94.1
Other non-current assets	69.1		0.0
Non-current assets held for sale			0.0
Total other assets	1,909.3		5,203.7
Total assets	1,909.3		5,205.5
Current liabilities
Accounts payable	0.0		0.0
Employee compensation and benefits	0.0		0.0
Other current liabilities	0.4		0.4
Current liabilities held for sale			0.0
Total current liabilities	0.4		0.4
Other liabilities
Long-term debt	0.0		0.0
Pension and other post-retirement compensation and benefits	0.0		0.0
Deferred tax liabilities	0.0		0.0
Other non-current liabilities	0.0		0.0
Non-current liabilities held for sale			0.0
Total liabilities	0.4		0.4
Equity
Total equity	1,908.9		5,205.1
Total liabilities and equity	1,909.3		5,205.5
Subsidiary Issuer
Current assets
Cash and cash equivalents	0.1		0.0		0.3		0.0
Accounts and notes receivable, net	1.8		0.0
Inventories	0.0		0.0
Other current assets	7.1		1.6
Current assets held for sale			0.0
Total current assets	9.0		1.6
Property, plant and equipment, net	0.0		0.0
Other assets
Investments in subsidiaries	2,476.5		7,156.1
Goodwill	0.0		0.0
Intangibles, net	0.0		0.0
Long-term intercompany debt			614.0
Other non-current assets	672.9		0.0
Non-current assets held for sale			0.0
Total other assets	3,149.4		7,770.1
Total assets	3,158.4		7,771.7
Current liabilities
Accounts payable	0.0		0.0
Employee compensation and benefits	0.0		0.0
Other current liabilities	4.3		9.5
Current liabilities held for sale			0.0
Total current liabilities	4.3		9.5
Other liabilities
Long-term debt	1,314.1		2,652.8
Pension and other post-retirement compensation and benefits	0.0		0.0
Deferred tax liabilities	0.0		0.0
Other non-current liabilities	0.0		0.0
Non-current liabilities held for sale			0.0
Total liabilities	1,318.4		2,662.3
Equity
Total equity	1,840.0		5,109.4
Total liabilities and equity	3,158.4		7,771.7
Non-Guarantor Subsidiaries
Current assets
Cash and cash equivalents	64.5		86.3		80.9		216.9
Accounts and notes receivable, net	400.6		483.1
Inventories	387.3		356.9
Other current assets	124.5		109.5
Current assets held for sale			708.0
Total current assets	976.9		1,743.8
Property, plant and equipment, net	274.2		279.8
Other assets
Investments in subsidiaries	0.0		0.0
Goodwill	2,097.0		2,112.8
Intangibles, net	289.4		321.8
Long-term intercompany debt			(708.1)
Other non-current assets	726.8		2,159.4
Non-current assets held for sale			3,989.6
Total other assets	3,113.2		7,875.5
Total assets	4,364.3		9,899.1
Current liabilities
Accounts payable	259.3		320.1
Employee compensation and benefits	84.5		115.4
Other current liabilities	325.0		301.0
Current liabilities held for sale			360.8
Total current liabilities	668.8		1,097.3
Other liabilities
Long-term debt	817.6		720.2
Pension and other post-retirement compensation and benefits	109.8		96.4
Deferred tax liabilities	106.3		108.6
Other non-current liabilities	185.1		183.3
Non-current liabilities held for sale			537.0
Total liabilities	1,887.6		2,742.8
Equity
Total equity	2,476.7		7,156.3
Total liabilities and equity	4,364.3		9,899.1
Consolidation, Eliminations
Current assets
Cash and cash equivalents	0.0		0.0		$ 0.0		$ 0.0
Accounts and notes receivable, net	0.0		0.0
Inventories	0.0		0.0
Other current assets	(10.7)		(9.2)
Current assets held for sale			0.0
Total current assets	(10.7)		(9.2)
Property, plant and equipment, net	0.0		0.0
Other assets
Investments in subsidiaries	(6,225.6)		(17,470.8)
Goodwill	0.0		0.0
Intangibles, net	0.0		0.0
Long-term intercompany debt			0.0
Other non-current assets	(1,332.9)		(1,980.7)
Non-current assets held for sale			0.0
Total other assets	(7,558.5)		(19,451.5)
Total assets	(7,569.2)		(19,460.7)
Current liabilities
Accounts payable	0.0		0.0
Employee compensation and benefits	0.0		0.0
Other current liabilities	(10.7)		(9.2)
Current liabilities held for sale			0.0
Total current liabilities	(10.7)		(9.2)
Other liabilities
Long-term debt	(1,332.9)		(1,980.7)
Pension and other post-retirement compensation and benefits	0.0		0.0
Deferred tax liabilities	0.0		0.0
Other non-current liabilities	0.0		0.0
Non-current liabilities held for sale			0.0
Total liabilities	(1,343.6)		(1,989.9)
Equity
Total equity	(6,225.6)		(17,470.8)
Total liabilities and equity	$ (7,569.2)		$ (19,460.7)