Employee Benefit Plans - Assumptions used (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension | Retirement Income Plan
Weighted-average assumptions used to determine obligations (balance sheet):
Discount rate	2.10%	3.00%	3.95%
Rate of compensation increase			3.50%
Weighted-average assumptions used to determine net periodic benefit cost (statement of operations):
Discount rate	3.00%	3.95%	3.35%
Expected return on plan assets	6.50%	6.50%	7.00%
Rate of compensation increase		3.50%	3.50%
Pension | National Service-Related Pension Plan
Weighted-average assumptions used to determine obligations (balance sheet):
Discount rate	2.49%	3.25%	4.15%
Weighted-average assumptions used to determine net periodic benefit cost (statement of operations):
Discount rate	3.25%	4.15%	3.65%
Expected return on plan assets	6.50%	6.50%	7.00%
Pension | Superannuation
Weighted-average assumptions used to determine obligations (balance sheet):
Discount rate	1.50%	2.30%	3.70%
Rate of compensation increase	3.25%	3.25%	3.25%
Weighted-average assumptions used to determine net periodic benefit cost (statement of operations):
Discount rate	2.30%	3.70%	3.70%
Expected return on plan assets	5.00%	5.00%	6.00%
Rate of compensation increase	3.25%	3.25%	4.00%
Other Post-Retirement Benefits
Weighted-average assumptions used to determine obligations (balance sheet):
Discount rate	1.41%	2.55%	3.70%
Weighted-average assumptions used to determine net periodic benefit cost (statement of operations):
Discount rate	2.55%	3.70%	3.10%