Employees, Retirement and Incentive Plans Retirement and Benefit Plan Costs (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) shares	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) Components shares	Jun. 30, 2019 USD ($)
Share-based Compensation Arrangements
Employee benefits and share-based compensation | $	$ 1.6	$ 1.7		$ 3.7
Long-term Incentive Plan, Components | Components			4
Number of incentive-based award plans	2		2
Compensation costs of incentive awards | $	$ 0.5	$ 0.6	$ 1.0	$ 1.2
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	1,119,542		1,119,542