Investment Securities (Rollforward of Fair Value Measurements by Level) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	$ 2,328.7	$ 3,208.3
Net realized and unrealized gains (losses)	245.6	(45.4)
Amortization/Accretion	(0.3)	(1.2)
Purchases	217.6	692.6
Sales	(257.0)	(1,321.8)
Transfers in	10.9	0.0
Transfers out	(10.9)	0.0
Ending balance	2,534.6	2,532.5
Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	842.6	890.4
Net realized and unrealized gains (losses)	91.7	(14.3)
Amortization/Accretion	0.1	0.0
Purchases	14.6	238.9
Sales	(149.7)	(240.7)
Transfers in	0.0	0.0
Transfers out	0.0	0.0
Ending balance	799.3	874.3
Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	1,160.5	2,105.4
Net realized and unrealized gains (losses)	40.7	(26.0)
Amortization/Accretion	(0.4)	(1.2)
Purchases	142.3	407.7
Sales	(105.4)	(1,080.8)
Transfers in	0.0	0.0
Transfers out	0.0	0.0
Ending balance	1,237.7	1,405.1
Other long-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	138.7	77.2
Net realized and unrealized gains (losses)	114.0	(7.8)
Amortization/Accretion	0.0	0.0
Purchases	11.5	44.0
Sales	0.0
Transfers in	10.9	0.0
Transfers out	0.0	0.0
Ending balance	275.1	113.4
Hedge and private equity funds included in other long-term investments | NAV
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	186.9	135.3
Net realized and unrealized gains (losses)	(0.8)	2.7
Amortization/Accretion	0.0	0.0
Purchases	49.2	2.0
Sales	(1.9)	(0.3)
Transfers in	0.0	0.0
Transfers out	(10.9)	0.0
Ending balance	222.5	139.7
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Realized and unrealized losses		(0.6)
Foreign Exchange Forward
Debt and Equity Securities, FV-NI [Line Items]
Realized and unrealized losses		(3.5)
Foreign Currency on Cash and Open Trades
Debt and Equity Securities, FV-NI [Line Items]
Realized and unrealized losses		(4.2)
Fair value measured on a recurring basis
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	2,542.9
Ending balance	2,804.4
Fair value measured on a recurring basis | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	1,047.0
Ending balance	1,048.3
Fair value measured on a recurring basis | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	1,170.3
Ending balance	1,258.5
Fair value measured on a recurring basis | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	138.7
Ending balance	275.1
Fair value measured on a recurring basis | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	138.7
Ending balance	275.1
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Ending balance	0.0
Fair value measured on a recurring basis | Other long-term investments | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Ending balance	0.0
Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	138.7
Ending balance	275.1
Fair value measured on a recurring basis | Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	269.8		$ 214.2
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	249.0	$ 763.0	204.4	$ 176.1
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	20.8		9.8
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	0.0		$ 0.0
Forward Contracts | Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Derivative, fair value, net				$ (3.7)
MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Unrealized investment gain	$ 114.7