Benefit Plans - Deferred Compensation Arrangements (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental pension arrangements for key executives
Benefit Plans
Employee's eligibility age to receive a defined monthly benefit	65 years
Accrual for present value of future benefit payments	$ 3.6	$ 3.6
Expenses related to the plan	0.0	0.2	$ 0.2
Deferred benefits for certain former key executives | Central
Benefit Plans
Accrual for present value of future benefit payments	2.6	2.7
Expenses related to the plan	$ (0.2)	0.6	$ 0.1
Minimum period over which the annual payments will be made when the executives retire or upon death or disability	10 years
Face value of life insurance contracts	$ 6.7
Cash surrender value of life insurance contracts	$ 4.0	$ 3.9