Employee Benefit Plans - Assumptions used (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Post-Retirement Benefits
Weighted-average assumptions used to determine obligations (balance sheet):
Discount rate	3.70%	3.10%	3.40%
Weighted-average assumptions used to determine net periodic benefit cost (statement of operations):
Discount rate	3.10%	3.40%	3.50%
Pension | Retirement Income Plan
Weighted-average assumptions used to determine obligations (balance sheet):
Discount rate	3.95%	3.35%	3.75%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase	3.50%	3.50%	3.50%
Weighted-average assumptions used to determine net periodic benefit cost (statement of operations):
Discount rate	3.35%	3.75%	4.00%
Expected return on plan assets	7.00%	7.00%	7.50%
Rate of compensation increase	3.50%	3.50%	3.50%
Pension | National Service-Related Pension Plan
Weighted-average assumptions used to determine obligations (balance sheet):
Discount rate	4.15%	3.65%	4.15%
Weighted-average assumptions used to determine net periodic benefit cost (statement of operations):
Discount rate	3.65%	4.15%	4.50%
Expected return on plan assets	7.00%	7.00%	7.50%
Pension | Superannuation
Weighted-average assumptions used to determine obligations (balance sheet):
Discount rate	3.70%	3.70%	4.20%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase	3.25%	4.00%	4.00%
Weighted-average assumptions used to determine net periodic benefit cost (statement of operations):
Discount rate	3.70%	4.20%	3.90%
Expected return on plan assets	6.00%	6.00%	6.50%
Rate of compensation increase	4.00%	4.00%	4.00%