Postretirement Benefit Plans - Fair Value of Plan Assets Held by Our Postretirement Benefits Plans (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	$ 326.7	$ 269.2	$ 286.9
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	321.5	263.1
Pension Plan [Member] | U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	70.6	67.4
Pension Plan [Member] | International [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	26.6	58.9
Pension Plan [Member] | Fixed income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	218.7	135.0
Pension Plan [Member] | Mutual Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	0.0	0.0
Pension Plan [Member] | Cash and other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	5.6	1.8
Pension Plan [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	5.6	1.8
Pension Plan [Member] | Level 1 [Member] | U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	0.0	0.0
Pension Plan [Member] | Level 1 [Member] | International [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	0.0	0.0
Pension Plan [Member] | Level 1 [Member] | Fixed income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	0.0	0.0
Pension Plan [Member] | Level 1 [Member] | Mutual Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	0.0	0.0
Pension Plan [Member] | Level 1 [Member] | Cash and other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	5.6	1.8
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	5.2	6.1	$ 7.9
Other Postretirement Benefits Plan [Member] | U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	0.0	0.0
Other Postretirement Benefits Plan [Member] | International [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	0.0	0.0
Other Postretirement Benefits Plan [Member] | Fixed income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	0.0	0.0
Other Postretirement Benefits Plan [Member] | Mutual Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	5.2	6.1
Other Postretirement Benefits Plan [Member] | Cash and other [Member]
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	5.2	6.1
Other Postretirement Benefits Plan [Member] | Level 1 [Member] | U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	0.0	0.0
Other Postretirement Benefits Plan [Member] | Level 1 [Member] | International [Member]
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 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	0.0	0.0
Other Postretirement Benefits Plan [Member] | Level 1 [Member] | Mutual Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	5.2	6.1
Other Postretirement Benefits Plan [Member] | Level 1 [Member] | Cash and other [Member]
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	315.9	261.3
Postretirement Assets Measured at Net Asset Value [Member] | Pension Plan [Member] | U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	70.6	67.4
Postretirement Assets Measured at Net Asset Value [Member] | Pension Plan [Member] | International [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	26.6	58.9
Postretirement Assets Measured at Net Asset Value [Member] | Pension Plan [Member] | Fixed income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	218.7	135.0
Postretirement Assets Measured at Net Asset Value [Member] | Pension Plan [Member] | Mutual Fund [Member]
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 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan asset	$ 0.0	$ 0.0