CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ (36.6)	$ (51.4)	$ 542.3	$ 209.4
Other comprehensive (loss) income:
Foreign currency translation gains (losses)	24.2	21.7	22.2	(24.4)
Net unrealized (loss) gain on derivative instruments
Changes in derivative instruments before reclassification adjustment	(18.4)	(10.4)	(19.2)	32.8
Net reclassification adjustment for losses (gains) included in net income	3.1	(3.8)	5.7	(8.6)
Changes in derivative instruments after reclassification adjustment	(15.3)	(14.2)	(13.5)	24.2
Changes in deferred income tax asset/liability	3.7	4.5	3.7	(9.7)
Net unrealized (loss) gain on hedging derivative instruments	(11.6)	(9.7)	(9.8)	14.5
Actuarial adjustments to pension plans
Changes in actuarial adjustments before reclassification adjustment	(1.5)	(0.8)	(2.2)	2.4
Net reclassification adjustment	0.8	1.3	1.6	2.6
Changes in actuarial adjustments to pension plans	(0.7)	0.5	(0.6)	5.0
Changes in deferred income tax asset/liability	0.2	0.2	0.2	(1.1)
Deferred tax valuation allowance adjustments	0.0	(0.1)	0.0	0.0
Net actuarial adjustments to pension plans	(0.5)	0.6	(0.4)	3.9
Unrealized investment gains (losses):
Changes in unrealized investment gains (losses) before reclassification adjustment	0.0	314.5	26.0	(352.5)
Net reclassification adjustment for gains included in net income	0.0	(1.8)	(6.3)	(3.9)
Changes in unrealized investment gains (losses) after reclassification adjustment	0.0	312.7	19.7	(356.4)
Adjustments to intangible assets	0.0	(100.1)	(0.9)	125.2
Changes in deferred income tax asset/liability	0.0	(74.6)	(6.7)	80.1
Net unrealized gains (losses) on investments	0.0	138.0	12.1	(151.1)
Deconsolidation of Insurance Operations	0.0	0.0	(445.9)	0.0
Non-credit related other-than-temporary impairment:
Net change to derive comprehensive income (loss) for the period	12.1	150.6	(421.8)	(157.1)
Comprehensive (loss) income	(24.5)	99.2	120.5	52.3
Less: Net income attributable to noncontrolling interest	0.5	30.7	72.0	79.3
Less: Other comprehensive income (loss) attributable to noncontrolling interest	5.1	32.0	7.6	(31.8)
Comprehensive income attributable to noncontrolling interest	5.6	62.7	79.6	47.5
Comprehensive (loss) income attributable to the controlling interest	$ (30.1)	$ 36.5	$ 40.9	$ 4.8