Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair value of financial instruments
Short-term investments	$ 0	$ 0
Level 1 Inputs
Fair value of financial instruments
Fair value measurement	84,456	90,560
Level 1 Inputs | Equity securities
Fair value of financial instruments
Fair value measurement	84,456	90,560
Level 2 Inputs
Fair value of financial instruments
Fair value measurement	1,154,269	1,081,637
Level 2 Inputs | U.S. Treasury securities
Fair value of financial instruments
Fair value measurement	302	1,801
Level 2 Inputs | Obligations of states and political subdivisions
Fair value of financial instruments
Fair value measurement	390,433	397,922
Level 2 Inputs | Residential mortgage-backed securities
Fair value of financial instruments
Fair value measurement	252,631	241,456
Level 2 Inputs | Commercial mortgage-backed securities
Fair value of financial instruments
Fair value measurement	35,454	28,663
Level 2 Inputs | Other asset-backed securities
Fair value of financial instruments
Fair value measurement	70,410	23,931
Level 2 Inputs | Corporate and other securities
Fair value of financial instruments
Fair value measurement	405,039	387,864
Level 3 Inputs
Fair value of financial instruments
Fair value measurement	678	547
Level 3 Inputs | Equity securities
Fair value of financial instruments
Fair value measurement	678	547
Total
Fair value of financial instruments
Fair value measurement	1,239,403	1,172,744
Total | U.S. Treasury securities
Fair value of financial instruments
Fair value measurement	302	1,801
Total | Obligations of states and political subdivisions
Fair value of financial instruments
Fair value measurement	390,433	397,922
Total | Residential mortgage-backed securities
Fair value of financial instruments
Fair value measurement	252,631	241,456
Total | Commercial mortgage-backed securities
Fair value of financial instruments
Fair value measurement	35,454	28,663
Total | Other asset-backed securities
Fair value of financial instruments
Fair value measurement	70,410	23,931
Total | Corporate and other securities
Fair value of financial instruments
Fair value measurement	405,039	387,864
Total | Equity securities
Fair value of financial instruments
Fair value measurement	$ 85,134	$ 91,107