Employee Benefit Plans - Other Postretirement Benefits (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Estimated future benefit payments 2018	$ 17.8
Estimated future benefit payments 2019	2.2
Estimated future benefit payments 2020	2.2
Estimated future benefit payments 2021	2.1
Estimated future benefit payments 2022	2.1
Estimated future benefit payments, five years thereafter	$ 9.0
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate used to measure net periodic benefit cost	4.16%	4.39%	4.01%
Discount rate used to measure accumulated postretirement benefit obligation	3.65%	4.15%
Estimated future benefit payments 2018	$ 0.4
Estimated future benefit payments 2019	0.4
Estimated future benefit payments 2020	0.4
Estimated future benefit payments 2021	0.4
Estimated future benefit payments 2022	0.4
Estimated future benefit payments, five years thereafter	$ 2.1