Employee benefit plans - Components of Net Periodic Pension Expense (Income) and Other Amounts Recognized in Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Changes in plan assets and benefit obligations recognized in other comprehensive income (loss):
Net loss (gain)	$ 7,567	$ 3,883
Pension Benefits [Member]
Components of net periodic pension cost:
Interest cost	678	646	673
Expected return on plan assets	(910)	(825)	(809)
Amortization of unrecognized net gain loss	92	180	136
Net periodic benefit cost	(140)	1
Changes in plan assets and benefit obligations recognized in other comprehensive income (loss):
Net loss (gain)	3,684	(2,369)	739
Deferred tax	(1,289)	805	(251)
Total recognized in other comprehensive income	2,395	(1,564)	488
Total recognized in net period pension cost and other comprehensive income	2,255	(1,563)	488
Postretirement Life Insurance Benefits [Member]
Components of net periodic pension cost:
Service cost		35	48
Interest cost		111	139
Amortization of unrecognized net gain loss		96	111
Net periodic benefit cost		242	298
Changes in plan assets and benefit obligations recognized in other comprehensive income (loss):
Net loss (gain)		(1,273)	185
Deferred tax		620	(63)
Total recognized in other comprehensive income		(653)	122
Total recognized in net period pension cost and other comprehensive income		$ (411)	$ 420

Amount Recognized In Net Periodic Benefit Cost And Other Comprehensive Income Loss Net Of Tax

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- Definition

Defined Benefit Plan Amortization Of Unrecognized Gains Losses

+ References

No definition available.

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- Definition

Defined benefit plan other comprehensive income loss deferred tax.

+ References

No definition available.

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- Definition

Amount before tax, after reclassification adjustments, of actuarial gain (loss) that has not been recognized in the income statement.

+ References

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- Definition

An amount calculated as a basis for determining the extent of delayed recognition of the effects of changes in the fair value of assets. The expected return on plan assets is determined based on the expected long-term rate of return on plan assets and the market-related value of plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 30

-Glossary Expected Return on Plan Assets

-URI http://asc.fasb.org/extlink&oid=6512136

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 60

-Glossary Expected Return on Plan Assets

-URI http://asc.fasb.org/extlink&oid=6512171

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (h)(3)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920

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- Definition

The increase in a defined benefit pension plan's projected benefit obligation or a defined benefit postretirement plan's accumulated postretirement benefit obligation due to the passage of time.

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- Definition

The total amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments.

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- Definition

The actuarial present value of benefits attributed by the pension benefit formula to services rendered by employees during the period. The portion of the expected postretirement benefit obligation attributed to employee service during the period. The service cost component is a portion of the benefit obligation and is unaffected by the funded status of the 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 (h)(1)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Service Cost (Component of Net Periodic Pension Cost)

-URI http://asc.fasb.org/extlink&oid=6525008

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920

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- Definition

Amount after tax and reclassification adjustments, of (increase) decrease in accumulated other comprehensive (income) loss related to pension and other postretirement defined benefit plans.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 220

-SubTopic 10

-Section 45

-Paragraph 10A

-Subparagraph (i-k)

-URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669646-108580

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