Benefit Plans - Deferred Compensation Arrangements (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
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	$ 3.7	$ 5.0
Expenses related to the plan	0.2	0.4	$ 0.1
Deferred benefits for certain former key executives | Central
Benefit Plans
Accrual for present value of future benefit payments	3.8	4.2
Expenses related to the plan	$ 0.1	1.0	$ 0.6
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	$ 6.9	10.8
Cash surrender value of life insurance contracts	$ 3.9	$ 3.9