Benefit Plans - Obligations and Funded Status (Details) - Pension Plan, Defined Benefit, Non-qualified - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015
Change in Benefit Obligation [Roll Forward]
Benefit obligation	$ 88.1	$ 96.6
Interest cost	4.1	4.1	$ 4.0
Benefits paid	(5.3)	(5.3)
Actuarial gain on obligation	0.6	(7.3)
Benefit obligation	87.5	88.1	96.6
Amounts Recognized in Consolidated Balance Sheets [Abstract]
Accrued benefit cost	$ (88.1)	$ (96.6)	$ (96.6)	$ (87.5)	$ (88.1)
Accumulated other comprehensive income:
Prior service cost				(2.5)	(3.7)
Net amount recognized				$ (90.0)	$ (91.8)