Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net income	$ 35.1	$ 102.1	$ 353.7
Other comprehensive income (loss), net of tax:
Unrealized gains on Investment in CyrusOne, net of tax of $4.4, $36.9	8.3	68.1	0.0
Reclassification adjustment for gain on sale of Investment in CyrusOne included in net income, net of tax of ($41.3)	(76.4)	0.0	0.0
Foreign currency translation gain (loss)	0.2	(0.1)	(0.4)
Defined benefit plans:
Net gain (loss) arising from remeasurement during the period, net of tax of $0.8, $3.6, ($3.4)	2.8	6.6	(6.6)
Amortization of prior service benefits included in net income, net of tax of ($1.6), ($5.2), ($5.5)	(2.9)	(9.4)	(9.8)
Amortization of net actuarial loss included in net income, net of tax of $7.9, $8.5, $10.8	14.3	15.5	19.5
Reclassification adjustment for pension settlement charges included in net income, net of tax of $1.5	2.5	0.0	0.0
Reclassification adjustment for curtailment loss included in net income, net of tax of $0.1	0.0	0.0	0.2
Other comprehensive income (loss)	(51.2)	80.7	2.9
Total comprehensive (loss) income	$ (16.1)	$ 182.8	$ 356.6