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Investment Securities (Rollforward of Fair Value Measurements by Level) (Details) - USD ($) | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2019 | Mar. 31, 2019 | Sep. 30, 2019 | Sep. 30, 2018 | Dec. 31, 2018 | Dec. 31, 2017
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  | $ 2,328,700,000 | $ 2,328,700,000 | $ 3,208,300,000 |  |
Net realized and unrealized gains |  |  | 347,500,000 | 31,300,000 |  |
Amortization/Accretion |  |  | (1,000,000.0) | (2,200,000) |  |
Purchases |  |  | 750,400,000 | 1,151,900,000 |  |
Sales |  |  | (856,200,000) | (2,024,200,000) |  |
Transfers in |  |  | 82,600,000 | 0 |  |
Transfers out |  |  | (82,600,000) | 0 |  |
Ending balance | $ 2,569,400,000 |  | 2,569,400,000 | 2,365,100,000 |  |
Level 1 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  | 842,600,000 | 842,600,000 | 890,400,000 |  |
Net realized and unrealized gains |  |  | 121,800,000 | 51,900,000 |  |
Amortization/Accretion |  |  | 200,000 | 100,000 |  |
Purchases |  |  | 101,400,000 | 412,600,000 |  |
Sales |  |  | (362,300,000) | (457,600,000) |  |
Transfers in |  |  | 0 | 0 |  |
Transfers out |  |  | 0 | 0 |  |
Ending balance | 703,700,000 |  | 703,700,000 | 897,400,000 |  |
Level 2 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  | 1,160,500,000 | 1,160,500,000 | 2,105,400,000 |  |
Net realized and unrealized gains |  |  | 66,600,000 | (33,600,000) |  |
Amortization/Accretion |  |  | (1,200,000) | (2,300,000) |  |
Purchases |  |  | 331,300,000 | 673,500,000 |  |
Sales |  |  | (465,200,000) | (1,565,000,000.0) |  |
Transfers in |  |  | 0 | 0 |  |
Transfers out |  |  | 0 | 0 |  |
Ending balance | 1,092,000,000.0 |  | 1,092,000,000.0 | 1,178,000,000.0 |  |
Other long-term investments | Level 3 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  | 138,700,000 | 138,700,000 | 77,200,000 |  |
Net realized and unrealized gains |  |  | 154,200,000 | (5,300,000) |  |
Amortization/Accretion |  |  | 0 | 0 |  |
Purchases |  |  | 211,300,000 | 55,100,000 |  |
Sales |  |  | 0 |  |  |
Transfers in |  |  | 82,600,000 | 0 |  |
Transfers out |  |  | 0 | 0 |  |
Ending balance | 586,800,000 |  | 586,800,000 | 127,000,000.0 |  |
Hedge and private equity funds included in other long-term investments | NAV |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  | 186,900,000 | 186,900,000 | 135,300,000 |  |
Net realized and unrealized gains |  |  | 4,900,000 | 18,300,000 |  |
Amortization/Accretion |  |  | 0 | 0 |  |
Purchases |  |  | 106,400,000 | 10,700,000 |  |
Sales |  |  | (28,700,000) | (1,600,000) |  |
Transfers in |  |  | 0 | 0 |  |
Transfers out |  |  | (82,600,000) | 0 |  |
Ending balance | 186,900,000 |  | 186,900,000 | 162,700,000 |  |
Short-term investments |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Short-term investments, at fair value | 272,500,000 |  | 272,500,000 |  | $ 214,200,000 |
Realized and unrealized losses |  |  | (300,000) | (700,000) |  |
Foreign Exchange Forward |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Realized and unrealized losses |  |  |  | (3,500,000) |  |
Foreign Currency on Cash and Open Trades |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Realized and unrealized losses |  |  |  | (4,200,000) |  |
Fair value measured on a recurring basis |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  | 2,542,900,000 | 2,542,900,000 |  |  |
Ending balance | 2,841,900,000 |  | 2,841,900,000 |  |  |
Fair value measured on a recurring basis | Level 1 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  | 1,047,000,000.0 | 1,047,000,000.0 |  |  |
Ending balance | 964,700,000 |  | 964,700,000 |  |  |
Fair value measured on a recurring basis | Level 2 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  | 1,170,300,000 | 1,170,300,000 |  |  |
Ending balance | 1,103,500,000 |  | 1,103,500,000 |  |  |
Fair value measured on a recurring basis | Level 3 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  | 138,700,000 | 138,700,000 |  |  |
Ending balance | 586,800,000 |  | 586,800,000 |  |  |
Fair value measured on a recurring basis | Other long-term investments |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  | 138,700,000 | 138,700,000 |  |  |
Ending balance | 586,800,000 |  | 586,800,000 |  |  |
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 |  | 138,700,000 | 138,700,000 |  |  |
Ending balance | 586,800,000 |  | 586,800,000 |  |  |
Fair value measured on a recurring basis | Short-term investments |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Short-term investments, at fair value | 272,500,000 |  | 272,500,000 |  | 214,200,000 |
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 | 261,000,000.0 |  | 261,000,000.0 | $ 313,700,000 | 204,400,000 | $ 176,100,000
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 | 11,500,000 |  | 11,500,000 |  | 9,800,000 |
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 |
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 |  |  |  |  |  | $ (3,700,000)
MediaAlpha |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Unrealized investment gain | $ 35,300,000 | $ 114,700,000 | 150,000,000.0 |  |  |
Kudu |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Purchases |  |  | 70,100,000 |  |  |
Transfers in |  |  | $ 71,700,000 |  |  |

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