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Employee Benefit Plans (Pension Benefit Plan) - Additional Information (Detail) - USD ($) $ in Millions | 12 Months Ended |
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Dec. 31, 2017 | Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligation | $ 32.9 | $ 28.7
Estimated amortization from accumulated other comprehensive income into net periodic pension cost in 2018 | $ 0.2 |
Bond Funds [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Percentage of target allocation ranges by asset class | 75.00% |
Equity Funds [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Percentage of target allocation ranges by asset class | 25.00% |
Minimum [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Threshold for unamortized gain losses | 10.00% |

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- Definition Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level. + 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 (e) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount included in accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan expected to be recognized in net periodic benefit (cost) credit for fiscal year following most recent annual statement of financial position. + 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 (s) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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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 + Details
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