Employee Benefit Plans (Narrative) (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Expense related to benefit plans	$ 12,700,000	$ 11,600,000	$ 11,300,000
Net actuarial loss expected to be recognized	$ 5,000,000
Modeling process confidence ratio	90.00%
Percentage of equity investments in mutual fund	65.00%
Net periodic benefit cost, Expected return on plan assets	7.25%	7.25%	7.25%
Expected defined benefit plans contribution within a year	$ 1,100,000
Target percentage of pay at age 55		45.00%
Target percentage of pay at age 60		60.00%
Contribution match by employer	100.00%
Participants contribution eligible for match	6.00%
Employer expense related to 401(k) plan	$ 14,300,000	$ 13,600,000	$ 13,300,000
Number of stock plans	3
Share-based compensation arrangement by share-based payment award, number of shares available for grant	1,367,750
Intrinsic value of outstanding in-the-money stock options	$ 91,300,000
Outstanding in-the-money exercisable stock options	75,800,000
Dividends, share-based compensation	$ 0.57	$ 0.54
Weighted-average risk-free interest rate	1.73%	1.00%	2.05%
Weighted-average fair value of stock options granted			$ 10.23
Dividend yield			3.02%
Weighted-average expected market price volatility			26.48%
Weighted-average expected term			5 years 8 months 12 days
Performance Shares [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Options awarded during period	24,162	29,240
Award payout percentage at less than 25th percentile level of achievement	0.00%
Award payout percentage at the 25th percentile level of achievement	50.00%
Award payout percentage at the 50th percentile level of achievement	100.00%
Award payout percentage at the 75th percentile level of achievement	150.00%
Employee Stock Option [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Options awarded during period	0	0	890,940
Equity Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Target asset allocation percentage	65.00%
Fixed Income Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Target asset allocation percentage	32.00%
Two Thousand Fifteen Plan [Member] | Stock Options [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Life period of options awarded, years	10 years	10 years
Vesting period of options awarded, years	4 years	4 years
Two Thousand Fifteen Plan [Member] | Non Vested Stock Awards and Stock Units [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Life period of options awarded, years	10 years	10 years
Cliff vesting period, years	4 years	4 years
Two Thousand Five Plan [Member] | Stock Options [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Life period of options awarded, years			10 years
Vesting period of options awarded, years			4 years
Two Thousand Five Plan [Member] | Non Vested Stock Awards and Stock Units [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Life period of options awarded, years			10 years
Cliff vesting period, years			4 years
2007 Directors Plan [Member] | Stock Options [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Options awarded during period	0	0	0
Minimum [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Contribution percentage of eligible compensation	2.00%
Maximum [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Contribution percentage of eligible compensation	50.00%
Maximum [Member] | Performance Shares [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Options awarded during period	36,243	43,860
Weighted-average fair value of stock options granted	$ 79.91	$ 69.70