Employee Benefit Plans (Asset Strategy Allocation) (Details)	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	100.00%	100.00%
Domestic Equity
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	30.00%	28.00%
International Equity
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	24.00%	21.00%
Private Equity
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	9.00%	8.00%
Total Equity
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	63.00%	57.00%
Target Percentage of Plan Assets by Strategy	70.00%
Domestic Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	16.00%	16.00%
High Yield Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	10.00%	12.00%
Total Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	26.00%	28.00%
Target Percentage of Plan Assets by Strategy	17.00%
Real Estate
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	5.00%	5.00%
Target Percentage of Plan Assets by Strategy	5.00%
Other
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	6.00%	10.00%
Target Percentage of Plan Assets by Strategy	8.00%
Maximum
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	100.00%
Maximum | Domestic Equity
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	40.00%
Maximum | International Equity
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	25.00%
Maximum | Private Equity
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	15.00%
Maximum | Total Equity
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	70.00%
Maximum | Domestic Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	40.00%
Maximum | High Yield Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	25.00%
Maximum | Total Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	65.00%
Maximum | Real Estate
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	15.00%
Maximum | Other
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	10.00%
Minimum | Domestic Equity
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	20.00%
Minimum | International Equity
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	10.00%
Minimum | Private Equity
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	0.00%
Minimum | Total Equity
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	40.00%
Minimum | Domestic Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	10.00%
Minimum | High Yield Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	0.00%
Minimum | Total Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	10.00%
Minimum | Real Estate
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	0.00%
Minimum | Other
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, total equity	0.00%