Employees, Retirement and Incentive Plans Retirement and Benefit Plan Costs (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($) shares	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) Components shares	Sep. 30, 2015 USD ($)
Share-based Compensation Arrangements
Employee benefits and share-based compensation | $	$ 1.4	$ 1.4	$ 4.3	$ 4.1
Long-term Incentive Plan, Components | Components			4
Compensation costs of incentive awards | $	$ 0.7	$ 1.3	$ 1.9	$ 2.9
Deferred Bonus [Member]
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	1,487,162		1,487,162