Employee Retirement Plans - Weighted Average Rate of Assumptions for Asset Allocation Plans (Details) |
12 Months Ended | ||
---|---|---|---|
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
|
Retirement Benefits [Abstract] | |||
Discount rate | 2.60% | 2.85% | 3.75% |
Actuarial valuation assumption used, increase in return on investment per year (as a percent) | 4.55% | ||
Rate of future RPI inflation | 3.15% | 3.20% | 3.05% |
Rate of future CPI inflation | 2.05% | 2.10% | 1.95% |
Pension increases in payment | 3.00% | 3.05% | 2.90% |
Long-term rate of return on assets | 4.01% | 3.17% | 4.50% |