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Reclassifications from Accumulated Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income Reclassification (Details) - USD ($) $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
(Loss)/income from operations before income tax	$ (195.1)	$ (263.0)	$ 77.0	$ 99.3	$ (74.1)	$ 100.5	$ 66.2	$ 116.9	$ 123.2	$ 30.0	$ 51.2	$ 133.1	$ (281.8)	$ 209.5	$ 337.5
Income tax expense	10.2	9.2	(1.2)	(2.8)	2.6	(4.9)	(1.3)	(2.5)	(5.3)	(1.8)	(2.2)	(5.1)	15.4	(6.1)	(14.4)
General, administrative and corporate expenses	(150.1)	(110.9)	(119.9)	(121.3)	(128.9)	(125.0)	(116.4)	(119.8)	(125.9)	(100.5)	(95.4)	(102.2)	(502.2)	(490.1)	(424.0)
Income (Loss), Including Portion Attributable to Noncontrolling Interest, before Tax													(266.4)	203.4	323.1
Net (loss)/income	$ (184.9)	$ (253.8)	$ 75.8	$ 96.5	$ (71.5)	$ 95.6	$ 64.9	$ 114.4	$ 117.9	$ 28.2	$ 49.0	$ 128.0	(266.4)	203.4	$ 323.1
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income (Loss), Including Portion Attributable to Noncontrolling Interest, before Tax													7.2	0.2
Available for sale securities \| Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Available for sale securities — gross realized gains													10.7	18.8
Realized (losses) on sale of securities													(6.7)	(8.9)
(Loss)/income from operations before income tax													4.0	9.9
Income tax expense													(0.4)	(1.0)
Net (loss)/income													3.6	8.9
Foreign currency translation adjustments \| Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense													(0.8)	0.0
General, administrative and corporate expenses													4.4	(8.7)
Income (Loss), Including Portion Attributable to Noncontrolling Interest, before Tax													$ 3.6	$ (8.7)