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Employee Benefit Plans - Additional Information (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Compensation And Retirement Disclosure [Abstract] |  |  |  |
Defined contribution pension plans |  | $ 2.7 | $ 2.5 | $ 2.4
CMRT |  |  |  |
Compensation And Retirement Disclosure [Abstract] |  |  |  |
Percentage of assets valued using observable inputs from level one or level two (or assets not subject to classification in the fair value hierarchy) |  | 92.00% | 91.00% |
Defined Benefit Pension Plans |  |  |  |
Compensation And Retirement Disclosure [Abstract] |  |  |  |
Expected contributions to Defined Benefit Plans in Next Fiscal Year |  | $ 0.3 |  |
Actuarial losses expected to be recognized as a component of net period cost in next fiscal year |  | $ 1.6 |  |
Weighted-average rate determining the actuarial present value of benefit obligation |  | 3.70% | 4.00% |
Long-term return on plan assets |  | 7.00% | 7.20% | 7.20%
Weighted average discount rate, periodic benefit cost |  | 4.00% | 3.80% | 4.50%
Defined Benefit Pension Plans | CMRT |  |  |  |
Compensation And Retirement Disclosure [Abstract] |  |  |  |
Long-term return on plan assets |  | 7.50% | 7.50% | 7.50%
OPEB |  |  |  |
Compensation And Retirement Disclosure [Abstract] |  |  |  |
Expected contributions to Defined Benefit Plans in Next Fiscal Year |  | $ 0.4 |  |
Weighted-average rate determining the actuarial present value of benefit obligation |  | 3.10% | 3.20% |
Eligible age for health care benefits |  | 65 years |  |
Weighted average discount rate, periodic benefit cost |  | 3.20% | 3.00% | 3.20%
OPEB | Scenario Forecast |  |  |  |
Compensation And Retirement Disclosure [Abstract] |  |  |  |
Actuarial losses expected to be recognized as a component of net period cost in next fiscal year | $ 0.2 |  |  |

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- Definition Reduced health benefit eligibility age. + References No definition available. + Details
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- Definition The interest rate used to adjust for the time value of money for 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 (k)(1) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Expected Long-Term Rate of Return on Plan Assets -URI http://asc.fasb.org/extlink&oid=6512046 Reference 2: 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=65897934&loc=d3e1928-114920 + Details
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- Definition Amount included in accumulated other comprehensive income (AOCI) related to gain (loss) expected to be recognized as components of net periodic benefit cost for the fiscal year that follows the most recent annual statement of financial position. + References No definition available. + Details
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- Definition Amount of employer's best estimate of contributions expected to be paid to the plan during the next fiscal year beginning after the date of the latest 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 (g) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition The amount of the cost recognized during the period for defined contribution plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942 + Details
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