Retirement Benefit Plans - Changes in Fair Value (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 93.9	$ 99.9
Purchases	7.6	12.6
Sales	18.1	22.4
Realized losses	(12.2)	(15.1)
Unrealized gains	9.4	18.9
Ending balance	80.6	93.9
Limited Partnerships [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	66.1	78.8
Purchases	0.6	2.1
Sales	(9.8)	16.8
Realized losses	(7.5)	(11.0)
Unrealized gains	3.4	13.0
Ending balance	52.8	66.1
Real Estate [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	27.8	21.1
Purchases	7.0	10.5
Sales	(8.3)	5.6
Realized losses	(4.7)	(4.1)
Unrealized gains	6.0	5.9
Ending balance	$ 27.8	$ 27.8