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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 |

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- Definition Defined benefit plan contribution employer match percentage + References No definition available. + Details
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- Definition Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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