Consolidated Statements of Comprehensive Income Parenthetical - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Unrealized gains on CyrusOne, tax	$ 36.9	$ 0.0	$ 0.0
Net gain (loss) arising from remeasurement during the period, tax	3.6	(3.4)	(25.0)
Amortization of prior service benefits included in net income, tax	(5.2)	(5.5)	(5.4)
Amortization of net actuarial loss included in net income, tax	8.5	10.8	8.0
Reclassification adjustment for curtailment loss included in net income, tax	$ 0.0	$ 0.1	$ 0.0