Accounting Policies - Intangible Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Gross carrying value non-amortizable	$ 25.8	$ 25.8
Gross carrying value amortizable	93.6	94.0
Accumulated amortization of intangible assets	35.0	27.3
Amortization expense of intangible assets	8.1	8.3	$ 10.3
Total Intangible Assets	$ 119.4	$ 119.8