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Investment Securities (Rollforward of Fair Value Measurements by Level) (Details) - USD ($) | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Mar. 31, 2019 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | $ 2,328,700,000 | $ 2,328,700,000 | $ 3,208,300,000 |
Total realized and unrealized gains (losses) |  | 433,000,000.0 | (99,800,000) |
Amortization/Accretion |  | (1,600,000) | (2,500,000) |
Purchases |  | 866,100,000 | 1,394,300,000 |
Sales |  | (950,300,000) | (2,171,600,000) |
Transfers in |  | 14,900,000 | 0 |
Transfers out |  | (14,900,000) | 0 |
Fair value investments |  | 2,746,000,000.0 | 2,328,700,000 |
Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 2,542,900,000 | 2,542,900,000 |  |
Fair value investments |  | 2,947,200,000 | 2,542,900,000 |
Other long-term investments | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 138,700,000 | 138,700,000 |  |
Fair value investments |  | 654,000,000.0 | 138,700,000 |
Short-term investments |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Securities, realized and unrealized gain (loss) |  | 200,000 | 800,000 |
Short-term investments | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value disclosure, investments |  |  |  | $ 176,100,000
Short-term investments, at amortized cost (which approximates fair value) |  | 201,200,000 | 214,200,000 |
Foreign Currency on Cash and Open Trades |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Securities, realized and unrealized gain (loss) |  |  | 4,200,000 |
Foreign Exchange Forward |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Securities, realized and unrealized gain (loss) |  |  | 3,500,000 |
Level 1 Inputs |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 842,600,000 | 842,600,000 | 890,400,000 |
Total realized and unrealized gains (losses) |  | 163,000,000.0 | (64,900,000) |
Amortization/Accretion |  | 200,000 | 200,000 |
Purchases |  | 165,300,000 | 514,700,000 |
Sales |  | (391,100,000) | (497,800,000) |
Transfers in |  | 0 | 0 |
Transfers out |  | 0 | 0 |
Fair value investments |  | 780,000,000.0 | 842,600,000 |
Level 1 Inputs | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 1,047,000,000.0 | 1,047,000,000.0 |  |
Fair value investments |  | 969,400,000 | 1,047,000,000.0 |
Level 1 Inputs | Common equity securities | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments |  | 547,500,000 |  |
Level 1 Inputs | Other long-term investments | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments |  | 0 |  |
Level 1 Inputs | Short-term investments | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Short-term investments, at amortized cost (which approximates fair value) |  | 189,400,000 | 204,400,000 |
Level 2 Inputs |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 1,160,500,000 | 1,160,500,000 | 2,105,400,000 |
Total realized and unrealized gains (losses) |  | 76,100,000 | (64,400,000) |
Amortization/Accretion |  | (1,800,000) | (2,700,000) |
Purchases |  | 404,500,000 | 783,800,000 |
Sales |  | (529,700,000) | (1,661,600,000) |
Transfers in |  | 0 | 0 |
Transfers out |  | 0 | 0 |
Fair value investments |  | 1,109,600,000 | 1,160,500,000 |
Level 2 Inputs | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 1,170,300,000 | 1,170,300,000 |  |
Fair value investments |  | 1,121,400,000 | 1,170,300,000 |
Level 2 Inputs | Common equity securities | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments |  | 136,300,000 |  |
Level 2 Inputs | Other long-term investments | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 0 | 0 |  |
Fair value investments |  | 0 | 0 |
Level 2 Inputs | Short-term investments | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Short-term investments, at amortized cost (which approximates fair value) |  | 11,800,000 | 9,800,000 |
Level 3 Inputs | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 138,700,000 | 138,700,000 |  |
Fair value investments |  | 654,100,000 | 138,700,000 |
Level 3 Inputs | Common equity securities |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 0 | 0 |  |
Total realized and unrealized gains (losses) |  | 0 |  |
Amortization/Accretion |  | 0 |  |
Purchases |  | 100,000 |  |
Sales |  | 0 |  |
Transfers in |  | 0 |  |
Transfers out |  | 0 |  |
Fair value investments |  | 100,000 | 0 |
Level 3 Inputs | Other long-term investments |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 138,700,000 | 138,700,000 | 77,200,000 |
Total realized and unrealized gains (losses) |  | 181,200,000 | 16,200,000 |
Amortization/Accretion |  | 0 | 0 |
Purchases |  | 185,400,000 | 45,300,000 |
Sales |  | 0 |  |
Transfers in |  | 10,900,000 | 0 |
Transfers out |  | (4,000,000.0) | 0 |
Fair value investments |  | 654,000,000.0 | 138,700,000 |
Level 3 Inputs | Other long-term investments | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 138,700,000 | 138,700,000 |  |
Fair value investments |  | 654,000,000.0 | 138,700,000 |
Level 3 Inputs | Hedge Funds and Private Equity Funds |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Amortization/Accretion |  |  | 0 |
Transfers in |  |  | 0 |
Transfers out |  |  | 0 |
Level 3 Inputs | Short-term investments | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Short-term investments, at amortized cost (which approximates fair value) |  | 0 | 0 |
Level 3 Inputs | Foreign Exchange Forward | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value disclosure, investments |  |  | (3,700,000) |
Fair Value Measured at Net Asset Value Per Share | Other long-term investments | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 186,900,000 | 186,900,000 |  |
Fair value investments |  |  | 186,900,000 |
Fair Value Measured at Net Asset Value Per Share | Hedge Funds and Private Equity Funds |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 186,900,000 | 186,900,000 | 135,300,000 |
Total realized and unrealized gains (losses) |  | 12,700,000 | 13,300,000 |
Amortization/Accretion |  | 0 |  |
Purchases |  | 110,800,000 | 50,500,000 |
Sales |  | (29,500,000) | (12,200,000) |
Transfers in |  | 4,000,000.0 |  |
Transfers out |  | (10,900,000) |  |
Fair value investments |  | 202,300,000 | $ 186,900,000 |
MediaAlpha |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Unrealized investment gain | $ 114,700,000 | 180,000,000.0 |  |
Kudu |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Effect of Kudu Transaction |  | 70,100,000 |  |
Kudu | Level 1 Inputs |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Effect of Kudu Transaction |  | 0 |  |
Kudu | Level 2 Inputs |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Effect of Kudu Transaction |  | 0 |  |
Kudu | Level 3 Inputs | Common equity securities |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Effect of Kudu Transaction |  | 0 |  |
Kudu | Level 3 Inputs | Other long-term investments |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Effect of Kudu Transaction |  | 141,800,000 |  |
Kudu | Fair Value Measured at Net Asset Value Per Share | Hedge Funds and Private Equity Funds |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Effect of Kudu Transaction |  | $ (71,700,000) |  |

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