Employees, Retirement and Incentive Plans Retirement and Benefit Plan Costs (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) shares	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) Components shares	Sep. 30, 2019 USD ($)
Share-based Compensation Arrangements
Employee benefits and share-based compensation | $	$ 1.9	$ 1.8	$ 5.7	$ 5.4
Long-term Incentive Plan, Components | Components			4
Number of incentive-based award plans	2		2
Compensation costs of incentive awards | $	$ 0.6	$ 0.5	$ 1.5	$ 1.8
Deferred Bonus
Share-based Compensation Arrangements
Units authorized under equity-based compensation plans (new) | shares	2,500,000		2,500,000
Number of units available for grant | shares	1,122,230		1,122,230