Employee Retirement Plans and Postretirement Benefits Target And Actual Allocations (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Equity funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target allocations (as a percent)		19.70%
Actual allocations (as a percent)		21.30%
Fixed Income Funds [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target allocations (as a percent)		65.90%
Actual allocations (as a percent)		64.10%
Hedge funds of funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target allocations (as a percent)		2.10%
Actual allocations (as a percent)		2.10%
Real estate funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target allocations (as a percent)		2.60%
Actual allocations (as a percent)		2.50%
Annuities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target allocations (as a percent)		3.00%
Actual allocations (as a percent)		3.00%
Other assets
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target allocations (as a percent)		6.70%
Actual allocations (as a percent)		7.00%
Move from equities to fixed income [Domain]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Target Allocation Percentage	0.05