Accumulated Other Comprehensive Income (Loss) - Reclasses Out Of AOCI (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance		$ 317.4
Other comprehensive income		12.2	$ 34.3
Cumulative effect of change in accounting principle	[1]	2.2
Ending balance		342.1
Benefit Plan Adjustments
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance		(601.0)
Other comprehensive income (loss) before reclassifications		(0.7)
Amounts reclassified from accumulated other comprehensive loss to net income (loss)		11.4
Other comprehensive income		10.7	6.8
Ending balance		(590.3)
Foreign Currency Translation Adjustments
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance		(327.4)
Other comprehensive income (loss) before reclassifications		0.1
Amounts reclassified from accumulated other comprehensive loss to net income (loss)		0.0
Other comprehensive income		0.1	26.3
Ending balance		(327.3)
Unrealized Gains (Losses) on Available-for-Sale Securities
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance		1.1
Other comprehensive income (loss) before reclassifications		0.0
Amounts reclassified from accumulated other comprehensive loss to net income (loss)		0.0
Other comprehensive income		0.0	0.1
Ending balance		0.0
Gains (Losses) on Cash Flow Hedges
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance		0.7
Other comprehensive income (loss) before reclassifications		0.3
Amounts reclassified from accumulated other comprehensive loss to net income (loss)		0.0
Other comprehensive income		0.3	0.0
Ending balance		1.0
AOCI Attributable to Parent
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance		(926.6)
Other comprehensive income (loss) before reclassifications		(0.3)
Amounts reclassified from accumulated other comprehensive loss to net income (loss)		11.4
Other comprehensive income		11.1	$ 33.2
Cumulative effect of change in accounting principle	[1]	(1.1)
Ending balance		(916.6)
Accounting Standards Update 2016-01 | Unrealized Gains (Losses) on Available-for-Sale Securities
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Cumulative effect of change in accounting principle		(1.1)
Accounting Standards Update 2016-01 | AOCI Attributable to Parent
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Cumulative effect of change in accounting principle		$ (1.1)

[1] Effective January 1, 2018, we adopted the provisions of ASU 2014-09, Revenue From Contracts with Customers, ASU 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities, and ASU 2016-16, Intra-Entity Transfers of Assets Other Than Inventory. We recognized a cumulative effect adjustment to January 1, 2018 retained earnings as a result of adopting each of these standards. See Note 1 for further details of the impact of each standard.