Employee Benefit Plans (Summary of Activity in Corporation's Active Stock Plans) (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Ending balance, shares available for grant	1,367,750
Stock options exercised, number of shares	(1,150,920)	(1,509,121)	(321,266)
Beginning balance, non-vested weighted-average grant-date fair value		$ 10.23
Ending balance, non-vested weighted-average grant-date fair value			$ 10.23
Director deferred stock units outstanding [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning balance, number of shares	53,659	45,443	38,867
Granted, number of shares	5,447	8,216	6,576
Stock awards exercised/vested, number of shares	(6,098)
Ending balance, number of shares	53,008	53,659	45,443
Beginning balance, Director Deferred, weighted-average grant-date fair value	$ 61.48	$ 61.35	$ 59.39
Granted, weighted-average grant-date fair value	95.37	63.25	72.94
Stock awards exercised/vested, weighted-average grant date fair value	62.29
Ending balance, Director Deferred, weighted-average grant-date fair value	$ 64.87	$ 61.48	$ 61.35
Non-Vested Stock Awards/ Stock Units Outstanding [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning balance, number of shares	256,850	173,180	175,490
Granted, number of shares	99,833	132,800	53,990
Stock awards exercised/vested, number of shares	(39,740)	(49,130)	(56,300)
Forfeited/expired, number of shares	(4,287)
Ending balance, number of shares	312,656	256,850	173,180
Beginning balance, non-vested weighted-average grant-date fair value	$ 73.43	$ 66.05	$ 60.55
Granted, weighted-average grant-date fair value	98.90	76.07	65.11
Stock awards exercised/vested, weighted-average grant date fair value	71.59	54.56	48.00
Ending balance, non-vested weighted-average grant-date fair value	81.71	$ 73.43	$ 66.05
Forfeited/expired, weighted-average exercise price	$ 79.52
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, number of shares	24,162	29,240
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning balance, number of shares	4,089,028	5,612,240	5,029,882
Granted, number of shares	0	0	890,940
Stock options exercised, number of shares	(1,118,122)	(1,476,841)	(287,326)
Forfeited/expired, number of shares	(53,764)	(46,371)	(21,256)
Ending balance, number of shares	2,917,142	4,089,028	5,612,240
Beginning balance, weighted-average exercise price	$ 62.67	$ 60.30	$ 58.99
Granted, weighted-average exercise price			65.11
Stock options exercised, weighted-average exercise price	60.59	53.40	51.70
Forfeited/expired, weighted-average exercise price	69.78	71.04	66.72
Ending balance, weighted-average exercise price	$ 63.34	$ 62.67	$ 60.30
Maximum [Member] | Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning balance, number of shares	43,860
Granted, number of shares	36,243	43,860
Ending balance, number of shares	80,103	43,860
Beginning balance, non-vested weighted-average grant-date fair value	$ 69.70
Granted, weighted-average grant-date fair value	92.27	$ 69.70
Ending balance, non-vested weighted-average grant-date fair value	$ 79.91	$ 69.70