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Retirement Plan, Based on Measurement Date (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Change in Plan Assets: |  |  |
Fair value of assets at end of year | $ 234,136 |  |
Pension Benefits |  |  |
Change in benefit obligation: |  |  |
Benefit obligation at beginning of year | 146,429 | $ 146,618 |
Interest cost | 5,616 | 5,881 | $ 6,063
Actuarial loss | 8,267 | 611 |
Annuity payments | (6,485) | (6,473) |
Settlements | (2,416) | (208) |
Benefit obligation at end of year | 151,411 | 146,429 | 146,618
Change in Plan Assets: |  |  |
Fair value of assets at beginning of year | 220,740 | 211,888 |
Actual return on plan assets | 22,297 | 15,533 |
Contributions | 0 | 0 |
Annuity payments | (6,485) | (6,473) |
Settlements | (2,416) | (208) |
Fair value of assets at end of year | 234,136 | 220,740 | $ 211,888
Funded status (included in "Other assets") | 82,725 | 74,311 |
Changes recognized in other comprehensive income (loss) for the year ended December 31: |  |  |
Amortization of prior service cost | 0 | 0 |
Amortization of actuarial loss | (8,209) | (9,050) |
Net actuarial loss arising during the year | 2,260 | 706 |
Total recognized in other comprehensive loss for the year (pre-tax) | (5,949) | (8,344) |
Accumulated other comprehensive loss (pre-tax) not yet recognized in net periodic benefit cost at December 31: |  |  |
Prior service cost | 0 | 0 |
Actuarial loss, net | 73,591 | 79,541 |
Total accumulated other comprehensive loss (pre-tax) | $ 73,591 | $ 79,541 |

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- Definition Amount, before tax, of accumulated other comprehensive (income) loss for defined benefit plan, that has not been recognized in net periodic benefit cost (credit). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580 Reference 2: 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=84228862&loc=SL7669646-108580 + Details
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- Definition Amount, before tax, of accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit. + 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 (j) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580 + Details
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- Definition Amount, before tax, of accumulated other comprehensive (income) loss for cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses. + 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 (b)(1) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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