Related Parties (Details) - FS&C [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Trinity [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Expenses from Transactions with Related Party			$ 0.1
Pension Plans, Defined Benefit [Member] | Investment Management Services for Defined Benefit Plan [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Expenses from Transactions with Related Party	$ 0.9	$ 0.8	0.4
Assets under Management, Carrying Amount	$ 171.8	$ 148.4	$ 137.2