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Fair Value Measurements (Details) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
U.S. Treasury securities and obligations of U.S. government corporations and agencies | $ 189,062 | $ 178,846
Obligations of U.S. states and political subdivisions | 2,102,697 | 2,152,524
Corporate debt securities | 2,056,079 | 2,066,838
ABS | 9,422 | 4,923
RMBS | 171,540 | 181,849
CMBS | 293,356 | 297,312
CLOs | 107,907 | 101,023
Total fixed income securities | 4,930,063 | 4,983,315
Equity securities | 4,099 | 7,246
Total investments at fair value | 4,934,162 | 4,990,561
Real estate acquired | 10,078 | 12,713
Quoted Prices in Active Markets for Identical Assets (Level 1) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
U.S. Treasury securities and obligations of U.S. government corporations and agencies | 81,418 | 81,598
Obligations of U.S. states and political subdivisions | 0 | 0
Corporate debt securities | 0 | 0
ABS | 0 | 0
RMBS | 0 | 0
CMBS | 0 | 0
CLOs | 0 | 0
Total fixed income securities | 81,418 | 81,598
Equity securities | 2,931 | 2,978
Total investments at fair value | 84,349 | 84,576
Real estate acquired | 0 | 0
Significant Other Observable Inputs (Level 2) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
U.S. Treasury securities and obligations of U.S. government corporations and agencies | 107,644 | 97,248
Obligations of U.S. states and political subdivisions | 2,102,443 | 2,152,253
Corporate debt securities | 2,056,079 | 2,066,838
ABS | 9,422 | 4,923
RMBS | 171,540 | 181,849
CMBS | 293,356 | 297,312
CLOs | 107,907 | 101,023
Total fixed income securities | 4,848,391 | 4,901,446
Equity securities | 0 | 0
Total investments at fair value | 4,848,391 | 4,901,446
Real estate acquired | 0 | 0
Significant Unobservable Inputs (Level 3) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
U.S. Treasury securities and obligations of U.S. government corporations and agencies | 0 | 0
Obligations of U.S. states and political subdivisions | 254 | 271
Corporate debt securities | 0 | 0
ABS | 0 | 0
RMBS | 0 | 0
CMBS | 0 | 0
CLOs | 0 | 0
Total fixed income securities | 254 | 271
Equity securities | 1,168 | 4,268
Total investments at fair value | 1,422 | 4,539
Real estate acquired | $ 10,078 | $ 12,713

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