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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%