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Retirement Benefit Plan - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Shares fully vested, Description	Shares allocated to a participant’s account are fully vested after three years of credited service, or in the event(s) of reaching age 65, death or disability while an active employee of the Company.
Shares fully vested after credited service	3 years
Common stock shares outstanding including ESOP	102,406,446		102,360,662
Company's stock held by ESOP	75,560,749		76,641,312
Direct Costs of Contracts and Selling, General and Administrative Expense
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
ESOP contribution expense	$ 13.2	$ 14.9

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- Definition

Common stock shares outstanding including employee stock ownership plan shares.

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The amount of plan compensation cost recognized during the period.

+ References

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-Publisher FASB
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-Topic 718
-SubTopic 40
-Section 50
-Paragraph 1
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Sum of the allocated, committed-to-be-released and suspense shares of the entity held by the plan.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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