Postretirement Benefit Plans - Fair Value of Plan Assets Held by Our Postretirement Benefits Plans (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	$ 321.8	$ 281.3	$ 326.7
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	320.5	278.4
Pension Plan [Member] | U.S. equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset		49.4
Pension Plan [Member] | International equities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset		25.1
Pension Plan [Member] | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	238.8	201.8
Pension Plan [Member] | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	0.0	0.0
Pension Plan [Member] | Cash and other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	81.7	2.1
Pension Plan [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	81.7	2.1
Pension Plan [Member] | Level 1 [Member] | U.S. equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset		0.0
Pension Plan [Member] | Level 1 [Member] | International equities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset		0.0
Pension Plan [Member] | Level 1 [Member] | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	0.0	0.0
Pension Plan [Member] | Level 1 [Member] | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	0.0	0.0
Pension Plan [Member] | Level 1 [Member] | Cash and other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	81.7	2.1
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	276.3
Pension Plan [Member] | Fair Value Measured at Net Asset Value Per Share [Member] | U.S. equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset		49.4
Pension Plan [Member] | Fair Value Measured at Net Asset Value Per Share [Member] | International equities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset		25.1
Pension Plan [Member] | Fair Value Measured at Net Asset Value Per Share [Member] | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	238.8	201.8
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	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	0.0
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	1.3	2.9	$ 5.2
Other Postretirement Benefits Plan [Member] | U.S. equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset		0.0
Other Postretirement Benefits Plan [Member] | International equities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset		0.0
Other Postretirement Benefits Plan [Member] | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	0.0	0.0
Other Postretirement Benefits Plan [Member] | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	1.3	2.9
Other Postretirement Benefits Plan [Member] | Cash and other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	0.0	0.0
Other Postretirement Benefits Plan [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	1.3	2.9
Other Postretirement Benefits Plan [Member] | Level 1 [Member] | U.S. equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset		0.0
Other Postretirement Benefits Plan [Member] | Level 1 [Member] | International equities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset		0.0
Other Postretirement Benefits Plan [Member] | Level 1 [Member] | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	0.0	0.0
Other Postretirement Benefits Plan [Member] | Level 1 [Member] | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	1.3	2.9
Other Postretirement Benefits Plan [Member] | Level 1 [Member] | Cash and other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	$ 0.0	$ 0.0