Fair Value Measurements - Fair Value Hierarchy for Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Millions | Sep. 30, 2019 | Dec. 31, 2018 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | $ 2,092 | $ 1,998 |
Total assets | 3,176 | 2,898 |
U.S. government and agencies | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 289 | 225 |
State and political subdivisions | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 77 | 148 |
Corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 1,596 | 1,503 |
Mortgage-backed/asset-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 72 | 60 |
Foreign government bonds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 58 | 62 |
Preferred securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities, at fair value | 284 | 301 |
Equity securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities, at fair value | 684 | 498 |
Other long-term investments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other long-term investments | 116 | 101 |
Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total assets | 702 | 514 |
Level 1 | U.S. government and agencies | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 0 | 0 |
Level 1 | State and political subdivisions | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 0 | 0 |
Level 1 | Corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 0 | 0 |
Level 1 | Mortgage-backed/asset-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 0 | 0 |
Level 1 | Foreign government bonds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 0 | 0 |
Level 1 | Preferred securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities, at fair value | 18 | 16 |
Level 1 | Equity securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities, at fair value | 684 | 498 |
Level 1 | Other long-term investments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other long-term investments | 0 | 0 |
Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total assets | 2,341 | 2,266 |
Level 2 | U.S. government and agencies | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 289 | 225 |
Level 2 | State and political subdivisions | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 77 | 148 |
Level 2 | Corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 1,579 | 1,486 |
Level 2 | Mortgage-backed/asset-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 72 | 60 |
Level 2 | Foreign government bonds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 58 | 62 |
Level 2 | Preferred securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities, at fair value | 266 | 285 |
Level 2 | Equity securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities, at fair value | 0 | 0 |
Level 2 | Other long-term investments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other long-term investments | 0 | 0 |
Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total assets | 133 | 118 |
Level 3 | U.S. government and agencies | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 0 | 0 |
Level 3 | State and political subdivisions | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 0 | 0 |
Level 3 | Corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 17 | 17 |
Level 3 | Mortgage-backed/asset-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 0 | 0 |
Level 3 | Foreign government bonds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 0 | 0 |
Level 3 | Preferred securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities, at fair value | 0 | 0 |
Level 3 | Equity securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities, at fair value | 0 | 0 |
Level 3 | Other long-term investments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other long-term investments | $ 116 | $ 101 |