Fair Value - Fair Value Measurements on a Recurring Basis by Asset Class and Level of Input (Details) | 9 Months Ended | 12 Months Ended |
Sep. 30, 2019USD ($) | Sep. 30, 2018USD ($) | Dec. 31, 2018USD ($)fund |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | $ 680,633,000 | | |
Equity securities | 55,052,000 | | $ 11,853,000 |
Other limited partnership investments | | | |
Distributions received from the funds | 3,600,000 | $ 3,046,000 | |
Contributions made to the funds | 20,000 | $ 217,000 | |
U.S. Treasury | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | | | 208,412,000 |
States & political subdivisions | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | | | 159,023,000 |
Corporate debt securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 434,206,000 | | 249,947,000 |
Residential mortgage-backed securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 107,235,000 | | 4,609,000 |
Commercial mortgage-backed securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 63,182,000 | | 46,515,000 |
Collateralized debt obligations | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 70,818,000 | | 64,239,000 |
Other debt securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 5,192,000 | | 15,778,000 |
Available-for-sale securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 680,633,000 | | 748,523,000 |
Fair Value, Measurements, Recurring Basis | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 680,633,000 | | |
Total | 735,685,000 | | 763,582,000 |
Fair Value, Measurements, Recurring Basis | U.S. Treasury | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | | | 208,412,000 |
Fair Value, Measurements, Recurring Basis | States & political subdivisions | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | | | 159,023,000 |
Fair Value, Measurements, Recurring Basis | Corporate debt securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 434,206,000 | | 249,947,000 |
Fair Value, Measurements, Recurring Basis | Residential mortgage-backed securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 107,235,000 | | 4,609,000 |
Fair Value, Measurements, Recurring Basis | Commercial mortgage-backed securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 63,182,000 | | 46,515,000 |
Fair Value, Measurements, Recurring Basis | Collateralized debt obligations | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 70,818,000 | | 64,239,000 |
Fair Value, Measurements, Recurring Basis | Other debt securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 5,192,000 | | 15,778,000 |
Fair Value, Measurements, Recurring Basis | Available-for-sale securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 680,633,000 | | 748,523,000 |
Fair Value, Measurements, Recurring Basis | Equity securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Equity securities | 55,052,000 | | 11,853,000 |
Fair Value, Measurements, Recurring Basis | Nonredeemable preferred stock | Financial services sector | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Equity securities | 47,753,000 | | $ 11,853,000 |
Fair Value, Measurements, Recurring Basis | Nonredeemable preferred stock | Industrial sector | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Equity securities | 2,842,000 | | |
Fair Value, Measurements, Recurring Basis | Nonredeemable preferred stock | Utilities sector | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Equity securities | 2,571,000 | | |
Fair Value, Measurements, Recurring Basis | Nonredeemable preferred stock | Energy sector | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Equity securities | 1,886,000 | | |
Fair Value, Measurements, Recurring Basis | Real estate | | | |
Other limited partnership investments | | | |
Number of real estate funds reported under the fair value option | fund | | | 1 |
Distributions received from the funds | 3,200,000 | | $ 1,200,000 |
Unfunded commitments | | | 0 |
Contributions made to the funds | | | 0 |
Fair Value, Measurements, Recurring Basis | Level 1 | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 3,214,000 | | |
Total | 28,738,000 | | 1,809,000 |
Fair Value, Measurements, Recurring Basis | Level 1 | U.S. Treasury | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | | | 0 |
Fair Value, Measurements, Recurring Basis | Level 1 | States & political subdivisions | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | | | 0 |
Fair Value, Measurements, Recurring Basis | Level 1 | Corporate debt securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 3,214,000 | | 0 |
Fair Value, Measurements, Recurring Basis | Level 1 | Residential mortgage-backed securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 0 | | 0 |
Fair Value, Measurements, Recurring Basis | Level 1 | Commercial mortgage-backed securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 0 | | 0 |
Fair Value, Measurements, Recurring Basis | Level 1 | Collateralized debt obligations | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 0 | | 0 |
Fair Value, Measurements, Recurring Basis | Level 1 | Other debt securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 0 | | 0 |
Fair Value, Measurements, Recurring Basis | Level 1 | Available-for-sale securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 3,214,000 | | 0 |
Fair Value, Measurements, Recurring Basis | Level 1 | Equity securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Equity securities | 25,524,000 | | 1,809,000 |
Fair Value, Measurements, Recurring Basis | Level 1 | Nonredeemable preferred stock | Financial services sector | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Equity securities | 25,003,000 | | 1,809,000 |
Fair Value, Measurements, Recurring Basis | Level 1 | Nonredeemable preferred stock | Industrial sector | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Equity securities | 0 | | |
Fair Value, Measurements, Recurring Basis | Level 1 | Nonredeemable preferred stock | Utilities sector | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Equity securities | 521,000 | | |
Fair Value, Measurements, Recurring Basis | Level 1 | Nonredeemable preferred stock | Energy sector | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Equity securities | 0 | | |
Fair Value, Measurements, Recurring Basis | Level 2 | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 663,354,000 | | |
Total | 692,882,000 | | 745,990,000 |
Fair Value, Measurements, Recurring Basis | Level 2 | U.S. Treasury | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | | | 208,412,000 |
Fair Value, Measurements, Recurring Basis | Level 2 | States & political subdivisions | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | | | 159,023,000 |
Fair Value, Measurements, Recurring Basis | Level 2 | Corporate debt securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 423,934,000 | | 237,370,000 |
Fair Value, Measurements, Recurring Basis | Level 2 | Residential mortgage-backed securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 107,235,000 | | 4,609,000 |
Fair Value, Measurements, Recurring Basis | Level 2 | Commercial mortgage-backed securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 58,475,000 | | 46,515,000 |
Fair Value, Measurements, Recurring Basis | Level 2 | Collateralized debt obligations | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 68,518,000 | | 64,239,000 |
Fair Value, Measurements, Recurring Basis | Level 2 | Other debt securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 5,192,000 | | 15,778,000 |
Fair Value, Measurements, Recurring Basis | Level 2 | Available-for-sale securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 663,354,000 | | 735,946,000 |
Fair Value, Measurements, Recurring Basis | Level 2 | Equity securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Equity securities | 29,528,000 | | 10,044,000 |
Fair Value, Measurements, Recurring Basis | Level 2 | Nonredeemable preferred stock | Financial services sector | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Equity securities | 22,750,000 | | 10,044,000 |
Fair Value, Measurements, Recurring Basis | Level 2 | Nonredeemable preferred stock | Industrial sector | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Equity securities | 2,842,000 | | |
Fair Value, Measurements, Recurring Basis | Level 2 | Nonredeemable preferred stock | Utilities sector | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Equity securities | 2,050,000 | | |
Fair Value, Measurements, Recurring Basis | Level 2 | Nonredeemable preferred stock | Energy sector | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Equity securities | 1,886,000 | | |
Fair Value, Measurements, Recurring Basis | Level 3 | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 14,065,000 | | |
Total | 14,065,000 | | 12,577,000 |
Fair Value, Measurements, Recurring Basis | Level 3 | U.S. Treasury | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | | | 0 |
Fair Value, Measurements, Recurring Basis | Level 3 | States & political subdivisions | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | | | 0 |
Fair Value, Measurements, Recurring Basis | Level 3 | Corporate debt securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 7,058,000 | | 12,577,000 |
Fair Value, Measurements, Recurring Basis | Level 3 | Residential mortgage-backed securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 0 | | 0 |
Fair Value, Measurements, Recurring Basis | Level 3 | Commercial mortgage-backed securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 4,707,000 | | 0 |
Fair Value, Measurements, Recurring Basis | Level 3 | Collateralized debt obligations | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 2,300,000 | | 0 |
Fair Value, Measurements, Recurring Basis | Level 3 | Other debt securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 0 | | 0 |
Fair Value, Measurements, Recurring Basis | Level 3 | Available-for-sale securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 14,065,000 | | 12,577,000 |
Fair Value, Measurements, Recurring Basis | Level 3 | Equity securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Equity securities | 0 | | 0 |
Fair Value, Measurements, Recurring Basis | Level 3 | Nonredeemable preferred stock | Financial services sector | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Equity securities | 0 | | 0 |
Fair Value, Measurements, Recurring Basis | Level 3 | Nonredeemable preferred stock | Industrial sector | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Equity securities | 0 | | |
Fair Value, Measurements, Recurring Basis | Level 3 | Nonredeemable preferred stock | Utilities sector | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Equity securities | 0 | | |
Fair Value, Measurements, Recurring Basis | Level 3 | Nonredeemable preferred stock | Energy sector | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Equity securities | $ 0 | | |
Fair Value, Measurements, Recurring Basis | Fair Value Measured at Net Asset Value Per Share | Real estate | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Other limited partnership investments | | | $ 3,206,000 |