Employee Benefits - Asset Target Allocations Prescribed by Trusts' Investment Policies (Details)	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	100.00%
Other Postretirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	100.00%
Equity Securities [Member] | Other Postretirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	80.00%
Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	81.00%
Fixed Income Securities [Member] | Other Postretirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	20.00%
Defined Benefit Plan, Cash and Cash Equivalents [Member] | Other Postretirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	0.00%