Employee Benefit Plans (Summary of Activity in Corporation's Active Stock Plans) (Detail) - $ / shares	12 Months Ended
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

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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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.

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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).

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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).

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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.

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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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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.

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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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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Gross number of share options (or share units) granted during the period.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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Number of share options (or share units) exercised during the current period.

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