Comprehensive Income (Loss) - Schedule of Changes in the Components of AOCI (Details) - USD ($) $ in Millions	6 Months Ended
Sep. 30, 2018	Mar. 31, 2018
Changes in the components of AOCI
Beginning balance	$ 3,279.8
Other comprehensive (loss) income before reclassifications	(3.5)
Amounts reclassified from accumulated other comprehensive loss	6.1
Net other comprehensive income (loss)	2.6
Ending balance	5,170.2
AOCI Attributable to Parent
Changes in the components of AOCI
Beginning balance	(17.6)
Impact of change in accounting principle		$ (1.7)
Opening Balance as of April 1, 2018		(19.3)
Ending balance	(16.7)
Unrealized holding gains (losses) available-for-sale debt securities
Changes in the components of AOCI
Beginning balance	1.9
Impact of change in accounting principle		(1.7)
Opening Balance as of April 1, 2018		0.2
Other comprehensive (loss) income before reclassifications	(5.6)
Amounts reclassified from accumulated other comprehensive loss	5.6
Net other comprehensive income (loss)	0.0
Ending balance	0.2
Defined benefit pension plans
Changes in the components of AOCI
Beginning balance	(10.1)
Impact of change in accounting principle		0.0
Opening Balance as of April 1, 2018		(10.1)
Other comprehensive (loss) income before reclassifications	3.5
Amounts reclassified from accumulated other comprehensive loss	0.5
Net other comprehensive income (loss)	4.0
Ending balance	(6.1)
Foreign Currency
Changes in the components of AOCI
Beginning balance	(9.4)
Impact of change in accounting principle		0.0
Opening Balance as of April 1, 2018		$ (9.4)
Other comprehensive (loss) income before reclassifications	(1.4)
Amounts reclassified from accumulated other comprehensive loss	0.0
Net other comprehensive income (loss)	(1.4)
Ending balance	$ (10.8)

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

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Amount after tax, before reclassification adjustments, of other comprehensive income (loss), attributable to parent.

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Amount after tax of other comprehensive income (loss) attributable to parent entity.

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Amount after tax of reclassification adjustments of other comprehensive income (loss) attributable to parent.

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Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

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Amount of stockholders' equity including the portion attributable to noncontrolling interests, after the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

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