Employee Retirement Plans - Weighted Average Rate of Assumptions for Asset Allocation Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2016
Retirement Benefits [Abstract]
Defined benefit plan assumptions retail price index inflation curve percentage			1.10%
Defined benefit plan, fixed interest gilt yield curve, percentage			0.70%
At the actuarial valuation date the market value of the defined benefit section’s assets			$ 173.3
Funded percentage			80.00%
Discount rate	3.00%	2.60%
Rate of future RPI inflation	3.15%	3.15%
Rate of future CPI inflation	2.05%	2.05%
Pension increases in payment	3.05%	3.00%
Long-term rate of return on assets	4.15%	3.99%