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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
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Amount of contributions received and expected to be received by defined benefit plan from employer in current fiscal year.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
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Amount of contributions expected to be received by defined benefit plan from employer in next fiscal year following latest fiscal year.

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Retirement Benefits - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Contributions by employer	$ 0.7
Minimum
Defined Benefit Plan Disclosure [Line Items]
Expected contributions in current fiscal year	5.0
Maximum
Defined Benefit Plan Disclosure [Line Items]
Expected contributions in current fiscal year	$ 20.0
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected contribution for next fiscal year		$ 5.5
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected contribution for next fiscal year		$ 0.1