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Employee Benefit Plans - Aggregate Fair Value of CMRT Assets (Detail) - CMRT - USD ($) $ in Millions | Dec. 31, 2016 | Dec. 31, 2015
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Defined Benefit Plan Disclosure [Line Items] |  |
CMRT asset value | $ 637.8 | $ 648.8
Assets not subject to fair value hierarchy | 30.00% | 30.00%
Total percentage of CMRT assets | 100.00% | 100.00%
Asset allocation percentage | 100.00% | 100.00%
Equities Principally Publicly Traded | United States |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Asset allocation percentage | 31.00% | 29.00%
Equities Principally Publicly Traded | Non-US |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Asset allocation percentage | 22.00% | 22.00%
Fixed Income Securities, Principally Publically Traded |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Asset allocation percentage | 36.00% | 38.00%
Privately Managed Limited Partnerships |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Asset allocation percentage | 5.00% | 5.00%
Hedge Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Asset allocation percentage | 5.00% | 5.00%
Other, Primarily Cash |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Asset allocation percentage | 1.00% | 1.00%
Level 1 |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Assets subject to fair value hierarchy | 54.00% | 54.00%
Level 2 |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Assets subject to fair value hierarchy | 8.00% | 7.00%
Level 3 |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Assets subject to fair value hierarchy | 8.00% | 9.00%

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