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Restricted Stock, Performance Share and Cash Awards - Schedule of Restricted Stock Awards Vesting Periods (Detail) - Restricted Stock Units Granted [Member] - shares | 3 Months Ended | 12 Months Ended
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Mar. 31, 2020 | Mar. 31, 2019 | Mar. 31, 2018 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Vesting Period [Line Items] |  |  |  |  |  |
Shares granted in the period | 405,870 | 399,900 | 420,200 | 422,610 | 414,700 | 439,100
Vesting Period One Year [Member] |  |  |  |  |  |
Vesting Period [Line Items] |  |  |  |  |  |
Shares granted in the period |  |  |  | 16,740 | 14,800 | 18,900
Vesting period, years |  |  |  | 1 year |  |
Vesting Period Two Years [Member] |  |  |  |  |  |
Vesting Period [Line Items] |  |  |  |  |  |
Shares granted in the period |  |  |  | 8,870 | 12,000 | 12,700
Vesting period, years |  |  |  | 2 years |  |
Vesting Period Five Years [Member] |  |  |  |  |  |
Vesting Period [Line Items] |  |  |  |  |  |
Shares granted in the period |  |  |  | 397,000 | 387,900 | 407,500
Vesting period, years |  |  |  | 5 years |  |

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- Definition Vesting Period [Line Items] + References No definition available. + Details
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- Definition 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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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