Qualified Employee Benefit Plans - The Retirement Plan's Asset Allocation Percentages (Details) - Retirement Plan [Member]	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, actual plan asset allocations, percent	100.00%	100.00%
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, actual plan asset allocations, percent	66.00%	61.00%
Debt Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, actual plan asset allocations, percent	15.00%	18.00%
Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, actual plan asset allocations, percent	19.00%	21.00%