Fair Value - Fair Value Measurements on a Recurring Basis by Asset Class and Level of Input (Details) | 6 Months Ended | 12 Months Ended |
Jun. 30, 2019USD ($) | Jun. 30, 2018USD ($) | Dec. 31, 2018USD ($)fund |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | $ 689,108,000 | | |
Equity securities | 12,445,000 | | $ 11,853,000 |
Other limited partnership investments | | | |
Distributions received from the funds | 2,450,000 | $ 2,682,000 | |
Contributions made to the funds | 9,000 | $ 215,000 | |
U.S. Treasury | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 151,094,000 | | 208,412,000 |
States & political subdivisions | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 3,360,000 | | 159,023,000 |
Corporate debt securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 356,656,000 | | 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 | 66,186,000 | | 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 | 46,142,000 | | 46,515,000 |
Collateralized debt obligations | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 57,345,000 | | 64,239,000 |
Other debt securities | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Available-for-sale securities | 8,325,000 | | 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 | 689,108,000 | | 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 | 689,108,000 | | |
Total | 701,553,000 | | 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 | 151,094,000 | | 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 | 3,360,000 | | 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 | 356,656,000 | | 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 | 66,186,000 | | 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 | 46,142,000 | | 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 | 57,345,000 | | 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 | 8,325,000 | | 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 | 689,108,000 | | 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 | 12,445,000 | | 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 | 12,445,000 | | $ 11,853,000 |
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 |
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 | 0 | | |
Total | 2,003,000 | | 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 | | 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 | | 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 | 0 | | 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 | 0 | | 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 | 2,003,000 | | 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 | 2,003,000 | | 1,809,000 |
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 | 680,184,000 | | |
Total | 690,626,000 | | 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 | 151,094,000 | | 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 | 3,360,000 | | 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 | 350,283,000 | | 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 | 66,186,000 | | 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 | 43,591,000 | | 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 | 57,345,000 | | 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 | 8,325,000 | | 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 | 680,184,000 | | 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 | 10,442,000 | | 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 | 10,442,000 | | 10,044,000 |
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 | 8,924,000 | | |
Total | 8,924,000 | | 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 | | 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 | | 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 | 6,373,000 | | 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 | 2,551,000 | | 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 | 0 | | 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 | 8,924,000 | | 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 | Fair value option using net asset value (NAV) | Real estate | | | |
Fair value measurements on a recurring basis by asset class and level of input | | | |
Other limited partnership investments | | | $ 3,206,000 |