Fair Value Measurements - Fair Value Hierarchy for Assets and Liabilities Measured on a Recurring Basis (Details) | Jun. 30, 2018USD ($)firm | Dec. 31, 2017USD ($) | Jun. 30, 2017USD ($) |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available for sale debt securities | $ 1,781,000,000 | $ 1,816,000,000 | |
Number of firms utilized | firm | 1 | | |
U.S. government and agencies | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available for sale debt securities | $ 245,000,000 | 195,000,000 | |
State and political subdivisions | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available for sale debt securities | 196,000,000 | 391,000,000 | |
Corporate debt securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available for sale debt securities | 1,230,000,000 | 1,117,000,000 | |
Mortgage-backed/asset-backed securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available for sale debt securities | 51,000,000 | 56,000,000 | |
Foreign government bonds | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available for sale debt securities | 59,000,000 | 57,000,000 | |
Preferred securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Equity securities, at fair value | 291,000,000 | 319,000,000 | |
Equity securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Equity securities, at fair value | 670,000,000 | 681,000,000 | |
Level 3 | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Assets held at fair value | | 0 | $ 0 |
Liabilities held at fair value | 0 | 0 | |
Fair Value, Measurements, Recurring | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets | 2,844,000,000 | 2,816,000,000 | |
Fair Value, Measurements, Recurring | U.S. government and agencies | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available for sale debt securities | 245,000,000 | 195,000,000 | |
Fair Value, Measurements, Recurring | State and political subdivisions | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available for sale debt securities | 196,000,000 | 391,000,000 | |
Fair Value, Measurements, Recurring | Corporate debt securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available for sale debt securities | 1,230,000,000 | 1,117,000,000 | |
Fair Value, Measurements, Recurring | Mortgage-backed/asset-backed securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available for sale debt securities | 51,000,000 | 56,000,000 | |
Fair Value, Measurements, Recurring | Foreign government bonds | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available for sale debt securities | 59,000,000 | 57,000,000 | |
Fair Value, Measurements, Recurring | Preferred securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Equity securities, at fair value | 291,000,000 | 319,000,000 | |
Fair Value, Measurements, Recurring | Equity securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Equity securities, at fair value | 670,000,000 | 681,000,000 | |
Fair Value, Measurements, Recurring | Other long-term investments | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Equity securities, at fair value | 102,000,000 | | |
Fair Value, Measurements, Recurring | Level 1 | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets | 687,000,000 | 704,000,000 | |
Fair Value, Measurements, Recurring | 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 | |
Fair Value, Measurements, Recurring | 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 | |
Fair Value, Measurements, Recurring | 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 | |
Fair Value, Measurements, Recurring | 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 | |
Fair Value, Measurements, Recurring | 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 | |
Fair Value, Measurements, Recurring | Level 1 | Preferred securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Equity securities, at fair value | 18,000,000 | 23,000,000 | |
Fair Value, Measurements, Recurring | Level 1 | Equity securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Equity securities, at fair value | 669,000,000 | 681,000,000 | |
Fair Value, Measurements, Recurring | Level 1 | Other long-term investments | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Equity securities, at fair value | 0 | | |
Fair Value, Measurements, Recurring | Level 2 | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets | 2,042,000,000 | 2,112,000,000 | |
Fair Value, Measurements, Recurring | 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 | 245,000,000 | 195,000,000 | |
Fair Value, Measurements, Recurring | Level 2 | State and political subdivisions | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available for sale debt securities | 196,000,000 | 391,000,000 | |
Fair Value, Measurements, Recurring | Level 2 | Corporate debt securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available for sale debt securities | 1,217,000,000 | 1,117,000,000 | |
Fair Value, Measurements, Recurring | 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 | 51,000,000 | 56,000,000 | |
Fair Value, Measurements, Recurring | Level 2 | Foreign government bonds | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available for sale debt securities | 59,000,000 | 57,000,000 | |
Fair Value, Measurements, Recurring | Level 2 | Preferred securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Equity securities, at fair value | 273,000,000 | 296,000,000 | |
Fair Value, Measurements, Recurring | Level 2 | Equity securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Equity securities, at fair value | 1,000,000 | 0 | |
Fair Value, Measurements, Recurring | Level 2 | Other long-term investments | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Equity securities, at fair value | 0 | | |
Fair Value, Measurements, Recurring | Level 3 | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets | 115,000,000 | 0 | |
Fair Value, Measurements, Recurring | 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 | |
Fair Value, Measurements, Recurring | 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 | |
Fair Value, Measurements, Recurring | Level 3 | Corporate debt securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available for sale debt securities | 13,000,000 | 0 | |
Fair Value, Measurements, Recurring | 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 | |
Fair Value, Measurements, Recurring | 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 | |
Fair Value, Measurements, Recurring | Level 3 | Preferred securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Equity securities, at fair value | 0 | 0 | |
Fair Value, Measurements, Recurring | Level 3 | Equity securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Equity securities, at fair value | 0 | $ 0 | |
Fair Value, Measurements, Recurring | Level 3 | Other long-term investments | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Equity securities, at fair value | $ 102,000,000 | | |