Retirement Plans - Reconciliation of Balances of Pension Benefit Obligation and Fair Value of Plan Assets (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan [Abstract]
Benefit obligation at beginning of year	$ 261.3	$ 261.8
Service cost	1.7	1.5	$ 1.1
Interest cost	10.0	10.8	10.8
Net actuarial loss	11.5	1.8
Benefits paid	(13.1)	(14.6)
Benefit obligation at end of year	271.4	261.3	261.8
Fair value of plan assets at beginning of year	207.8	207.5
Actual return on plan assets	24.7	14.9
Contributions by the company	0.0	0.0	0.0
Benefits paid	(13.1)	(14.6)
Fair value of plan assets at end of year	219.4	207.8	$ 207.5
Funded status of the plan (underfunded)	(52.0)	(53.5)
Noncurrent liabilities - accrued benefit liability	(52.0)	(53.5)
Accumulated other comprehensive loss - net actuarial loss	63.7	67.9
Net amount included in retained earnings	$ 11.7	$ 14.4

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- Definition

Amount, before tax, of accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

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Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses.

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Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive plan.

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Amount of asset (liability), recognized in statement of financial position, for defined benefit pension and other postretirement plans.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
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Amount of actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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Amount of payment to participant of defined benefit plan which decreases benefit obligation. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

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Amount of contribution received by defined benefit plan from employer which increases plan assets.

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Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

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Amount of funded (unfunded) status of defined benefit plan, measured as difference between fair value of plan assets and benefit obligation. Includes, but is not limited to, overfunded (underfunded) status.

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Amount of cost recognized for passage of time related to defined benefit plan.

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Amount of cost for actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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Amount of liability, recognized in statement of financial position, for defined benefit pension and other postretirement plans, classified as noncurrent.

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