Employee Retirement Plans - Target Asset Allocation and Acceptable Ranges (Details)	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations (as a percent)	100.00%
Equity
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations (as a percent)	36.00%
Fixed income
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations (as a percent)	54.00%
Limited partnerships
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations (as a percent)	5.00%
Other real assets
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations (as a percent)	5.00%
Cash equivalents and short-term investments
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations (as a percent)	0.00%
Minimum | Equity
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations (as a percent)	32.00%
Minimum | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations (as a percent)	45.00%
Minimum | Limited partnerships
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations (as a percent)	1.00%
Minimum | Other real assets
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations (as a percent)	2.00%
Minimum | Cash equivalents and short-term investments
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations (as a percent)	0.00%
Maximum | Equity
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations (as a percent)	40.00%
Maximum | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations (as a percent)	63.00%
Maximum | Limited partnerships
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations (as a percent)	8.00%
Maximum | Other real assets
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations (as a percent)	9.00%
Maximum | Cash equivalents and short-term investments
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations (as a percent)	5.00%