Fair Value Measurements (Details) - USD ($) | Sep. 30, 2016 | Dec. 31, 2015 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | $ 2,476,000,000 | $ 2,558,000,000 |
Total assets | 3,229,000,000 | 3,192,000,000 |
U.S. government and agencies | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 110,000,000 | 117,000,000 |
State and political subdivisions | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 617,000,000 | 768,000,000 |
Corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 1,571,000,000 | 1,495,000,000 |
Mortgage-backed/asset-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 63,000,000 | 71,000,000 |
Foreign government bonds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 115,000,000 | 107,000,000 |
Preferred stock available for sale | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities available for sale, at fair value | 321,000,000 | 289,000,000 |
Equity securities available for sale | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities available for sale, at fair value | 432,000,000 | 345,000,000 |
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 | 3,229,000,000 | 3,192,000,000 |
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 | 110,000,000 | 117,000,000 |
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 | 617,000,000 | 768,000,000 |
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 | 1,571,000,000 | 1,495,000,000 |
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 | 63,000,000 | 71,000,000 |
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 | 115,000,000 | 107,000,000 |
Fair Value, Measurements, Recurring | Preferred stock available for sale | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities available for sale, at fair value | 321,000,000 | 289,000,000 |
Fair Value, Measurements, Recurring | Equity securities available for sale | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities available for sale, at fair value | 432,000,000 | 345,000,000 |
Fair Value, Measurements, Recurring | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total assets | 467,000,000 | 376,000,000 |
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 stock available for sale | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities available for sale, at fair value | 35,000,000 | 42,000,000 |
Fair Value, Measurements, Recurring | Level 1 | Equity securities available for sale | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities available for sale, at fair value | 432,000,000 | 334,000,000 |
Fair Value, Measurements, Recurring | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total assets | 2,762,000,000 | 2,816,000,000 |
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 | 110,000,000 | 117,000,000 |
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 | 617,000,000 | 768,000,000 |
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 | 1,571,000,000 | 1,495,000,000 |
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 | 63,000,000 | 71,000,000 |
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 | 115,000,000 | 107,000,000 |
Fair Value, Measurements, Recurring | Level 2 | Preferred stock available for sale | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities available for sale, at fair value | 286,000,000 | 247,000,000 |
Fair Value, Measurements, Recurring | Level 2 | Equity securities available for sale | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities available for sale, at fair value | 0 | 11,000,000 |
Fair Value, Measurements, Recurring | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total assets | 0 | 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 | 0 | 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 stock available for sale | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities available for sale, at fair value | 0 | 0 |
Fair Value, Measurements, Recurring | Level 3 | Equity securities available for sale | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities available for sale, at fair value | $ 0 | $ 0 |