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Fair Value Measurements (Details) - USD ($) $ in Thousands | Sep. 30, 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 | $ 176,216 | $ 178,846
Obligations of U.S. states and political subdivisions | 1,734,606 | 2,152,524
Corporate debt securities | 2,255,065 | 2,066,838
ABS | 70,935 | 4,923
RMBS | 154,963 | 181,849
CMBS | 270,220 | 297,312
CLOs | 307,607 | 101,023
Total fixed income securities | 4,969,612 | 4,983,315
Equity securities | 7,720 | 7,246
Total investments at fair value | 4,977,332 | 4,990,561
Real estate acquired | 12,339 | 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 | 29,584 | 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 | 29,584 | 81,598
Equity securities | 2,889 | 2,978
Total investments at fair value | 32,473 | 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 | 146,632 | 97,248
Obligations of U.S. states and political subdivisions | 1,734,529 | 2,152,253
Corporate debt securities | 2,255,065 | 2,066,838
ABS | 70,935 | 4,923
RMBS | 154,963 | 181,849
CMBS | 270,220 | 297,312
CLOs | 307,607 | 101,023
Total fixed income securities | 4,939,951 | 4,901,446
Equity securities | 0 | 0
Total investments at fair value | 4,939,951 | 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 | 77 | 271
Corporate debt securities | 0 | 0
ABS | 0 | 0
RMBS | 0 | 0
CMBS | 0 | 0
CLOs | 0 | 0
Total fixed income securities | 77 | 271
Equity securities | 4,831 | 4,268
Total investments at fair value | 4,908 | 4,539
Real estate acquired | $ 12,339 | $ 12,713

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