Employees, Retirement and Incentive Plans Retirement and Benefit Plan Costs (Details) $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($) Components shares	Mar. 31, 2019 USD ($)
Share-based Compensation Arrangements
Employee benefits and share-based compensation | $	$ 2.2	$ 1.9
Long-term Incentive Plan, Components | Components	4
Number of incentive-based award plans	2
Compensation costs of incentive awards | $	$ 0.5	$ 0.7
Deferred Bonus
Share-based Compensation Arrangements
Units authorized under equity-based compensation plans (new) | shares	2,500,000
Number of units available for grant | shares	1,116,919