Fair Value Measurements (Details) - USD ($) $ in Thousands | Jun. 30, 2019 | Dec. 31, 2018 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | $ 5,504,823 | $ 5,151,987 |
Equity securities, at fair value (cost 2019 - $3,991; 2018 - $3,993) | 4,114 | 3,932 |
Other | 169,584 | 96,403 |
Real estate acquired | 10,250 | 14,535 |
Total assets | 5,688,771 | 5,266,857 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other | 169,584 | 96,403 |
Real estate acquired | 0 | 0 |
Total assets | 216,523 | 142,599 |
Significant Other Observable Inputs (Level 2) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other | 0 | 0 |
Real estate acquired | 0 | 0 |
Total assets | 5,461,998 | 5,109,710 |
Significant Unobservable Inputs (Level 3) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other | 0 | 0 |
Real estate acquired | 10,250 | 14,535 |
Total assets | 10,250 | 14,548 |
Total fixed income securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 5,504,823 | 5,151,987 |
Total fixed income securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 42,825 | 42,264 |
Total fixed income securities | Significant Other Observable Inputs (Level 2) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 5,461,998 | 5,109,710 |
Total fixed income securities | Significant Unobservable Inputs (Level 3) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 0 | 13 |
U.S. Treasury securities and obligations of U.S. government corporations and agencies | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 195,569 | 167,176 |
U.S. Treasury securities and obligations of U.S. government corporations and agencies | Quoted Prices in Active Markets for Identical Assets (Level 1) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 42,825 | 42,264 |
U.S. Treasury securities and obligations of U.S. government corporations and agencies | Significant Other Observable Inputs (Level 2) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 152,744 | 124,912 |
U.S. Treasury securities and obligations of U.S. government corporations and agencies | Significant Unobservable Inputs (Level 3) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 0 | 0 |
Obligations of U.S. states and political subdivisions | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 1,653,897 | 1,720,100 |
Obligations of U.S. states and political subdivisions | Quoted Prices in Active Markets for Identical Assets (Level 1) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 0 | 0 |
Obligations of U.S. states and political subdivisions | Significant Other Observable Inputs (Level 2) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 1,653,897 | 1,720,087 |
Obligations of U.S. states and political subdivisions | Significant Unobservable Inputs (Level 3) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 0 | 13 |
Corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 2,628,614 | 2,400,762 |
Corporate debt securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 0 | 0 |
Corporate debt securities | Significant Other Observable Inputs (Level 2) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 2,628,614 | 2,400,762 |
Corporate debt securities | Significant Unobservable Inputs (Level 3) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 0 | 0 |
Asset backed securities (“ABS”) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 217,701 | 112,033 |
Asset backed securities (“ABS”) | Quoted Prices in Active Markets for Identical Assets (Level 1) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 0 | 0 |
Asset backed securities (“ABS”) | Significant Other Observable Inputs (Level 2) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 217,701 | 112,033 |
Asset backed securities (“ABS”) | Significant Unobservable Inputs (Level 3) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 0 | 0 |
Residential mortgage backed securities (“RMBS”) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 208,497 | 178,961 |
Residential mortgage backed securities (“RMBS”) | Quoted Prices in Active Markets for Identical Assets (Level 1) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 0 | 0 |
Residential mortgage backed securities (“RMBS”) | Significant Other Observable Inputs (Level 2) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 208,497 | 178,961 |
Residential mortgage backed securities (“RMBS”) | Significant Unobservable Inputs (Level 3) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 0 | 0 |
Commercial mortgage backed securities (“CMBS”) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 271,889 | 267,660 |
Commercial mortgage backed securities (“CMBS”) | Quoted Prices in Active Markets for Identical Assets (Level 1) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 0 | 0 |
Commercial mortgage backed securities (“CMBS”) | Significant Other Observable Inputs (Level 2) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 271,889 | 267,660 |
Commercial mortgage backed securities (“CMBS”) | Significant Unobservable Inputs (Level 3) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 0 | 0 |
Collateralized loan obligations (“CLO”) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 328,656 | 305,295 |
Collateralized loan obligations (“CLO”) | Quoted Prices in Active Markets for Identical Assets (Level 1) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 0 | 0 |
Collateralized loan obligations (“CLO”) | Significant Other Observable Inputs (Level 2) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 328,656 | 305,295 |
Collateralized loan obligations (“CLO”) | Significant Unobservable Inputs (Level 3) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,357,436; 2018 - $5,196,784) | 0 | 0 |
Equity securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities, at fair value (cost 2019 - $3,991; 2018 - $3,993) | 4,114 | 3,932 |
Equity securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities, at fair value (cost 2019 - $3,991; 2018 - $3,993) | 4,114 | 3,932 |
Equity securities | Significant Other Observable Inputs (Level 2) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities, at fair value (cost 2019 - $3,991; 2018 - $3,993) | 0 | 0 |
Equity securities | Significant Unobservable Inputs (Level 3) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities, at fair value (cost 2019 - $3,991; 2018 - $3,993) | $ 0 | $ 0 |