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Employee Benefit Plans - Health Care Cost Trend Rate (Details)
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Retirement Benefits [Abstract]      
Pre-65 health care cost trend rate 6.75% 6.90% 7.30%
Post-65 health care cost trend rate     5.50%
Rate to which the cost trend rate is assumed to decline (the ultimate trend rate) 4.50% 5.00% 5.00%