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Employee Benefit Plans (Summary of Activity in Corporation's Active Stock Plans) (Detail) - $ / shares | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Stock options exercised, number of shares | (408,563) | (399,224) | (548,238)
Director deferred stock units outstanding [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Ending balance, number of shares/units | 52,860 | 55,370 | 48,910
Beginning balance, number of shares/units | 55,370 | 48,910 | 53,008
Ending balance, weighted-average grant-date fair value | $ 75.47 | $ 74.76 | $ 71.14
Beginning balance, weighted-average grant-date fair value | $ 74.76 | $ 71.14 | $ 64.87
Granted in period, number of shares/units | 10,428 | 7,592 | 6,576
Granted in period, weighted-average grant-date fair value | $ 73.84 | $ 102.70 | $ 109.58
Stock awards exercised/vested in period, number of shares | (12,938) | (1,132) | (10,674)
Stock awards exercised/vested in period, weighted-average grant date fair value | $ 71.09 | $ 106.03 | $ 63.68
Non-Vested Stock Awards/ Stock Units Outstanding [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Ending balance, number of shares/units | 470,359 | 440,647 | 383,797
Beginning balance, number of shares/units | 440,647 | 383,797 | 312,656
Ending balance, weighted-average grant-date fair value | $ 86.24 | $ 90.22 | $ 85.59
Beginning balance, weighted-average grant-date fair value | $ 90.22 | $ 85.59 | $ 81.71
Granted in period, number of shares/units | 151,038 | 127,091 | 109,847
Granted in period, weighted-average grant-date fair value | $ 66.79 | $ 93.46 | $ 94.81
Stock awards exercised/vested in period, number of shares | (117,990) | (53,990) | (32,050)
Stock awards exercised/vested in period, weighted-average grant date fair value | $ 76.07 | $ 65.11 | $ 78.92
Forfeited/expired in period, number of shares/units | (3,336) | (16,251) | (6,656)
Forfeited/expired in period, weighted-average grant date fair value | $ 91.07 | $ 89.71 | $ 87.60
Share-based Payment Arrangement, Option [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Ending balance, number of shares | 1,739,559 | 1,980,866 | 2,352,008
Beginning balance, number of shares | 1,980,866 | 2,352,008 | 2,917,142
Ending balance, weighted-average exercise price | $ 66.11 | $ 64.60 | $ 63.55
Beginning balance, weighted-average exercise price | $ 64.60 | $ 63.55 | $ 63.34
Stock options exercised, number of shares | (235,880) | (359,892) | (513,134)
Stock options exercised, weighted-average exercise price | $ 53.23 | $ 57.71 | $ 61.68
Forfeited/expired, number of shares | (5,427) | (11,250) | (52,000)
Forfeited/expired, weighted-average exercise price | $ 75.74 | $ 65.11 | $ 70.42
Options awarded during period | 0 | 0 | 0
Maximum [Member] | Performance Shares [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Ending balance, number of shares/units | 201,257 | 177,288 | 125,809
Beginning balance, number of shares/units | 177,288 | 125,809 | 80,106
Ending balance, weighted-average grant-date fair value | $ 77.18 | $ 83.48 | $ 82.55
Beginning balance, weighted-average grant-date fair value | $ 83.48 | $ 82.55 | $ 79.91
Granted in period, number of shares/units | 72,618 | 51,479 | 45,703
Granted in period, weighted-average grant-date fair value | $ 57.89 | $ 85.74 | $ 87.18
Stock awards exercised/vested in period, number of shares | (41,755) |  |
Stock awards exercised/vested in period, weighted-average grant date fair value | $ 69.70 |  |
Forfeited/expired in period, number of shares/units | (6,894) |  |
Forfeited/expired in period, weighted-average grant date fair value | $ 81.33 |  |

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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period. + 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)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event. + 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)(3) -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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- Definition The weighted average fair value at grant date for nonvested equity-based awards issued 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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- Definition The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date. + 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)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options. + 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)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period. + 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)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement. + 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)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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