Defined benefit plan number of common shares sold to ESOP trust.

No definition available.

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- Definition

Preceding year asset gain or loss considered to calculate expected return on plan assets.

+ References

No definition available.

+ Details

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- Definition

Defined benefit plan percentage of preceding year asset gain or loss considered to calculate expected return on plan assets.

+ References

No definition available.

+ Details

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- Definition

Defined benefit plan percentage of second preceding year asset gain or loss considered to calculate expected return on plan assets.

+ References

No definition available.

+ Details

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- Definition

Preceding year asset gain or loss considered to calculate expected return on plan assets.

+ References

No definition available.

+ Details

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- Definition

Amount of employer's best estimate of payments expected to be paid out of operating cash to plan participants during the next fiscal year beginning after the date of the latest statement of financial position.

+ References

No definition available.

+ Details

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- Definition

Percentage of expected long term rate of return on plan assets.

+ References

No definition available.

+ Details

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- Definition

Percentage of long term return in cash.

+ References

No definition available.

+ Details

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- Definition

Percentage of premium estimated return for equities and properties to risk free rate.

+ 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

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- Definition

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.

+ References

No definition available.

+ Details

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- Definition

Amount of contributions expected to be received by defined benefit plan from employer in next fiscal year following latest fiscal year.

+ 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 (g)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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- Definition

Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one 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 (d)(5)(i)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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Postretirement Benefit Plans - Defined Benefit Plans - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Nov. 20, 2017	Feb. 21, 2017	Feb. 23, 2016
Defined Benefit Plan Disclosure [Line Items]
Assets gains or losses added for prior year	80.00%
Assets gains or losses added for second preceding year	60.00%
Assets gains or losses added for third preceding year	40.00%
Assets gains or losses added for fourth preceding year	20.00%
United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Common shares sold to the Company's ESOP trust				18,827	40,837	56,894
Expected long-term rate of return on assets	7.00%	7.00%	7.50%
Expected payment related to qualified plan	$ 0
Expected payment related to non-qualified plan	$ 302,000
U.K Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected long-term rate of return on assets	3.77%	4.59%	4.66%
Percentage of premium estimated return for equities and properties to risk free rate	3.00%
Percentage of long term return in cash	4.00%
Expected payment related to qualified plan	$ 461,000
Equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Percentage of target allocation for investment category	50.00%
Expected long-term rate of return on assets	6.31%
Equities [Member] \| U.K Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Percentage of target allocation for investment category	58.00%
Debt Securities [Member] \| United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Percentage of target allocation for investment category	30.00%
Debt Securities [Member] \| U.K Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Percentage of target allocation for investment category	29.00%
Employer Securities [Member] \| United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Percentage of target allocation for investment category	20.00%
Insurance Contracts [Member] \| U.K Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Percentage of target allocation for investment category	7.00%
Real Estate Fund [Member] \| U.K Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Percentage of target allocation for investment category	3.00%
Cash [Member] \| U.K Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Percentage of target allocation for investment category	3.00%
U.S. and international equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected long-term rate of return on assets	4.69%