Leases - Narrative (Detail) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Operating lease costs	$ 4.0			$ 4.1			$ 11.8	$ 11.1
Amortization of right-of-use assets	0.4			0.4			1.3	1.3
Interest on lease liabilities	0.4			0.5			1.3	1.4
Variable lease costs	0.4			0.3			1.2	0.8
Total lease costs	5.3			5.2			15.6	14.6
Operating cash flows from operating leases							13.3	12.6
Operating cash flows from finance leases							1.3	1.4
Financing cash flows from finance leases							1.2	1.0
Non-cash amounts for lease liabilities arising from obtaining right-of-use assets, operating leases							3.3	1.0
Non-cash amounts for lease liabilities arising from obtaining right-of-use assets, finance leases							0.3	0.5
Operating lease right-of-use assets	66.0						66.0		$ 73.1
Finance lease assets	26.7						26.7		26.5
Total lease ROU assets	1,826.9						1,826.9		1,877.7
Current operating lease liabilities	14.9						14.9		13.9
Current finance lease liabilities	1.7						1.7		1.4
Liabilities, Current	239.6						239.6		280.4
Non-current operating lease liabilities	57.3						57.3		65.6
Non-current finance lease liabilities	19.5						19.5		20.6
Total lease liabilities	72.2						72.2		79.5
Finance Lease, Liability	21.2						21.2		22.0
TOTAL LIABILITIES	1,330.8						1,330.8		1,445.7
Net income	21.4	$ 22.8	$ 10.3	(11.0)	$ (0.4)	$ 3.8	54.5	(7.6)
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, after Tax	(1.8)			(1.3)			(5.5)	(3.9)
Other Comprehensive Income (Loss), Net of Tax	1.8			1.3			5.5	3.9
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	23.2			(9.7)			60.0	(3.7)
Lease assets [Member]
Capital Leases, Lessee Balance Sheet, Assets by Major Class, Accumulated Depreciation	(12.3)						(12.3)		(11.1)
Total lease ROU assets	80.4						80.4		88.5
Lease liabilities, current [Member]
Liabilities, Current	16.5						16.5		15.3
Lease liabilities, noncurrent [Member]
Liabilities, Noncurrent	76.8						76.8		86.2
Lease liabilities [Member]
TOTAL LIABILITIES	93.4						93.4		$ 101.5
Finance lease costs [Member]
Total lease costs	$ 0.9			$ 0.9			$ 2.7	$ 2.7
Minimum
Remaining lease term							1 year
Maximum
Remaining lease term							11 years