Retirement Plans - Allocation of Plan Assets, Pension Plan (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	$ 875.4	$ 3,424.1	$ 3,656.5
Total pension plan assets	875.4	3,424.1
Estimate of Fair Value Measurement | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	387.1	2,314.1
Estimate of Fair Value Measurement | Pension Benefits | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	17.5	124.0
Estimate of Fair Value Measurement | Pension Benefits | Equity
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	144.6	816.7
Estimate of Fair Value Measurement | Pension Benefits | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	222.7	1,371.9
Estimate of Fair Value Measurement | Pension Benefits | Derivatives and Other
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	2.3	1.5
Portion at Other than Fair Value Measurement | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at net asset value	488.3	1,110.0
Portion at Other than Fair Value Measurement | Pension Benefits | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at net asset value	193.4	184.4
Portion at Other than Fair Value Measurement | Pension Benefits | Derivatives and Other
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at net asset value	16.6	16.5
Portion at Other than Fair Value Measurement | Pension Benefits | Real Estate
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at net asset value	32.8	146.5
Portion at Other than Fair Value Measurement | Pension Benefits | Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at net asset value	245.5	717.5
Portion at Other than Fair Value Measurement | Private Equity
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at net asset value	0.0	45.1
Fair Value, Inputs, Level 1 | Estimate of Fair Value Measurement | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	158.1	910.5
Fair Value, Inputs, Level 1 | Estimate of Fair Value Measurement | Pension Benefits | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	12.9	93.3
Fair Value, Inputs, Level 1 | Estimate of Fair Value Measurement | Pension Benefits | Equity
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	144.5	816.6
Fair Value, Inputs, Level 1 | Estimate of Fair Value Measurement | Pension Benefits | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	0.7	0.6
Fair Value, Inputs, Level 1 | Estimate of Fair Value Measurement | Pension Benefits | Derivatives and Other
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	0.0	0.0
Fair Value, Inputs, Level 2 | Estimate of Fair Value Measurement | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	229.0	1,403.6
Fair Value, Inputs, Level 2 | Estimate of Fair Value Measurement | Pension Benefits | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	4.6	30.7
Fair Value, Inputs, Level 2 | Estimate of Fair Value Measurement | Pension Benefits | Equity
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	0.1	0.1
Fair Value, Inputs, Level 2 | Estimate of Fair Value Measurement | Pension Benefits | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	222.0	1,371.3
Fair Value, Inputs, Level 2 | Estimate of Fair Value Measurement | Pension Benefits | Derivatives and Other
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	2.3	1.5
Fair Value, Inputs, Level 3 | Estimate of Fair Value Measurement | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	0.0	0.0
Fair Value, Inputs, Level 3 | Estimate of Fair Value Measurement | Pension Benefits | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	0.0	0.0
Fair Value, Inputs, Level 3 | Estimate of Fair Value Measurement | Pension Benefits | Equity
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	0.0	0.0
Fair Value, Inputs, Level 3 | Estimate of Fair Value Measurement | Pension Benefits | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	0.0	0.0
Fair Value, Inputs, Level 3 | Estimate of Fair Value Measurement | Pension Benefits | Derivatives and Other
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	$ 0.0	$ 0.0