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Employee Benefits - Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Pension Costs |  |  |
Components of net periodic benefit cost (including amounts capitalized to the balance sheets) |  |  |
Service cost | $ 47.1 | $ 44.6 | $ 45.4
Interest cost | 114.3 | 121.8 | 130.8
Expected return on plan assets | (196.5) | (195.7) | (195.9)
Plan settlement | 1.0 | 9.0 | 16.5
Amortization of prior service cost (credit) | 2.7 | 2.9 | 3.4
Amortization of net actuarial loss | 94.0 | 86.1 | 82.9
Net periodic benefit cost (credit) | 62.6 | 68.7 | 83.1
OPEB Costs |  |  |
Components of net periodic benefit cost (including amounts capitalized to the balance sheets) |  |  |
Service cost | 23.7 | 24.1 | 26.1
Interest cost | 29.9 | 32.9 | 37.0
Expected return on plan assets | (59.5) | (55.5) | (52.7)
Plan settlement | 0.0 | 0.0 | 0.0
Amortization of prior service cost (credit) | (15.4) | (12.3) | (9.4)
Amortization of net actuarial loss | 1.0 | 3.1 | 8.5
Net periodic benefit cost (credit) | $ (20.3) | $ (7.7) | $ 9.5

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