Postretirement Benefit Plans - Fair Value of Plan Assets Held by Our Postretirement Benefits Plans (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	$ 0.5	$ 321.8	$ 281.3
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	0.5	320.5
Pension Plan [Member] | Fixed income - collective trusts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset		238.8
Pension Plan [Member] | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset		0.0
Pension Plan [Member] | Cash and other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset		81.7
Pension Plan [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset		81.7
Pension Plan [Member] | Level 1 [Member] | Fixed income - collective trusts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset		0.0
Pension Plan [Member] | Level 1 [Member] | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset		0.0
Pension Plan [Member] | Level 1 [Member] | Cash and other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset		81.7
Pension Plan [Member] | Fair Value Measured at Net Asset Value Per Share [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset		238.8
Pension Plan [Member] | Fair Value Measured at Net Asset Value Per Share [Member] | Fixed income - collective trusts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset		238.8
Pension Plan [Member] | Fair Value Measured at Net Asset Value Per Share [Member] | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset		0.0
Pension Plan [Member] | Fair Value Measured at Net Asset Value Per Share [Member] | Cash and other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset		0.0
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	$ 0.0	1.3	$ 2.9
Other Postretirement Benefits Plan [Member] | Fixed income - collective trusts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset		0.0
Other Postretirement Benefits Plan [Member] | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset		1.3
Other Postretirement Benefits Plan [Member] | Cash and other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset		$ 0.0