Postretirement Benefit Plans - Fair Value of Plan Assets Held by Our Postretirement Benefits Plans (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	$ 281.3	$ 326.7	$ 269.2
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	278.4	321.5
Pension Plan [Member] | U.S. equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	49.4	70.6
Pension Plan [Member] | International equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	25.1	26.6
Pension Plan [Member] | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	201.8	218.7
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	2.1	5.6
Pension Plan [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	2.1	5.6
Pension Plan [Member] | Level 1 [Member] | U.S. equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	0.0	0.0
Pension Plan [Member] | Level 1 [Member] | International equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	0.0	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	2.1	5.6
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	2.9	5.2	$ 6.1
Other Postretirement Benefits Plan [Member] | U.S. equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	0.0	0.0
Other Postretirement Benefits Plan [Member] | International equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	0.0	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	2.9	5.2
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	2.9	5.2
Other Postretirement Benefits Plan [Member] | Level 1 [Member] | U.S. equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	0.0	0.0
Other Postretirement Benefits Plan [Member] | Level 1 [Member] | International equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	0.0	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	2.9	5.2
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
Postretirement Assets Measured at Net Asset Value [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	276.3	315.9
Postretirement Assets Measured at Net Asset Value [Member] | Pension Plan [Member] | U.S. equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	49.4	70.6
Postretirement Assets Measured at Net Asset Value [Member] | Pension Plan [Member] | International equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	25.1	26.6
Postretirement Assets Measured at Net Asset Value [Member] | Pension Plan [Member] | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	201.8	218.7
Postretirement Assets Measured at Net Asset Value [Member] | Pension Plan [Member] | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	0.0	0.0
Postretirement Assets Measured at Net Asset Value [Member] | Pension Plan [Member] | Cash and other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	$ 0.0	$ 0.0