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Employee service share based compensation nonvested awards compensation cost expected to be recognized in year four.

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Employee service share based compensation nonvested awards compensation cost expected to be recognized in year three.

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Employee service share based compensation nonvested awards compensation cost expected to be recognized in year two.

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Minimum age to qualify for postretirement medical benefits.

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Minimum service required for vesting.

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Minimum year of service to qualify for postretirement medical benefits.

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Share based compensation arrangement by share based payment award equity instruments other than options vest in year three.

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Share based compensation arrangement by share based payment award equity instruments other than options vest in year two.

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Share based compensation arrangement by share based payment award equity instruments other than options vested in year four.

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Share based compensation arrangement by share based payment award percentage paid in cash.

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Share based compensation arrangement by share based payment award percentage paid in restricted stock.

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Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Amount of gain (loss) recognized in net periodic benefit (cost) credit from event reducing expected years of future service of present employees or eliminating accrual of defined benefits for some or all future services of present employees.

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Amount of cost for defined contribution plan.

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Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

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Percentage of employer's matching contributions to a defined contribution plan that vests in a given year.

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Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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 weighted average intrinsic value of equity-based compensation awards vested. Excludes stock and unit options.

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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 maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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

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The floor of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range.

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The ceiling of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range.

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term of outstanding stock options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Benefit Plans - Additional Information (Detail) - USD ($)		12 Months Ended
	Jul. 12, 1999	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Apr. 30, 2015	Apr. 26, 2007
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Minimum year of service to qualify for post-retirement medical benefits		20 years
Minimum age to qualify for post-retirement medical benefits		60 years
Condition to qualify for post-retirement medical benefits		Employees who worked 20 years and attained a minimum age of 60 by September 1, 2000
Gain on curtailments				$ 1,072,000
Unrealized gains, reduction of salaries and employee benefits			$ 1,200,000
Matching contribution percentage		50.00%	50.00%	50.00%
Maximum pre-tax contribution of the employees' base pay		6.00%	6.00%	6.00%
Percentage of vesting of participants in Home Savings contributions		100.00%
Minimum service required for vesting in Home Savings contribution		3 years
Expense related to the plan		$ 619,000	$ 480,000	$ 465,000
Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Option expiry term (Years)		10 years
Shares, Granted		0	0	6,618
Stock-based compensation expense		$ 1,000	$ 9,000
Weighted average remaining life for outstanding stock		2 years 6 months 21 days
Exercise price range, lower range limit		$ 1.20
Exercise price range, upper range limit		$ 5.89
Restricted Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense		$ 951,000	$ 970,000	$ 756,000
Nonvested shares		277,035	341,184
Vested shares during 2018		135,635
Vested shares during 2019		107,260
Vested shares during 2020		34,140
Expected additional expense for 2018		$ 784,000
Expected additional expense for 2019		362,000
Expected additional expense for 2020		$ 154,000
Total average per share fair value of shares vested		$ 8.61	$ 5.97	$ 5.48
2015 Long Term Incentive Compensation Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Authorized shares of stock, option plan, maximum					1,200,000
2007 Long-Term Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Authorized shares of stock, option plan, maximum						2,000,000
1999 Long-Term Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Authorized shares of stock, option plan, maximum		3,569,766
Option expiry term (Years)	10 years
1999 Long-Term Incentive Plan [Member] \| Stock Option [Member] \| Beginning on December 31, 2009 [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation vesting period	3 years
Annual Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of amount paid in cash		80.00%
Percentage of amount paid in restricted stock		20.00%
Annual Incentive Plan [Member] \| Restricted Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense		$ 315,000	$ 356,000	$ 269,000
Annual Incentive Plan [Member] \| Restricted Stock [Member] \| Beginning on first anniversary of issue date [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation vesting period		3 years
Annual Incentive Plan [Member] \| Cash Portion [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense		$ 1,700,000	1,300,000	804,000
Long Term Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense		$ 230,000	$ 693,000	$ 199,000