Qualified Employee Benefit Plans - The Retirement Plan's Asset Allocation Percentages (Details) - Retirement Plan	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, actual plan asset allocations, percent	100.00%	100.00%
Equity
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, actual plan asset allocations, percent	43.00%	66.00%
Debt securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, actual plan asset allocations, percent	41.00%	15.00%
Other
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, actual plan asset allocations, percent	16.00%	19.00%