Fair Value Measurements - Available-for-Sale Securities Measured at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financial Assets:
Total fixed-maturity securities	$ 237,484	$ 166,248
Equity securities	58,911	53,035
Total available-for-sale securities	296,395	219,283
Trading equity securities	1,045	0
Fair Value, Measurements, Recurring [Member]
Financial Assets:
Cash and cash equivalents	255,884	280,531
Total fixed-maturity securities	237,484	166,248
Equity securities	58,911	53,035
Total available-for-sale securities	296,395	219,283
Trading equity securities	1,045
Total financial assets	553,324	499,814
Fair Value, Measurements, Recurring [Member] | U.S. Treasury and U.S. Government Agencies [Member]
Financial Assets:
Total fixed-maturity securities	42,027	1,939
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]
Financial Assets:
Total fixed-maturity securities	107,103	74,504
Fair Value, Measurements, Recurring [Member] | State, Municipalities, and Political Subdivisions [Member]
Financial Assets:
Total fixed-maturity securities	80,695	78,306
Fair Value, Measurements, Recurring [Member] | Exchange-Traded Debt [Member]
Financial Assets:
Total fixed-maturity securities	7,659	11,262
Fair Value, Measurements, Recurring [Member] | Redeemable Preferred Stock [Member]
Financial Assets:
Total fixed-maturity securities		237
Fair Value, Measurements, Recurring [Member] | (Level 1) [Member]
Financial Assets:
Cash and cash equivalents	255,884	280,531
Total fixed-maturity securities	154,295	86,957
Equity securities	58,911	53,035
Total available-for-sale securities	213,206	139,992
Trading equity securities	1,045
Total financial assets	470,135	420,523
Fair Value, Measurements, Recurring [Member] | (Level 1) [Member] | U.S. Treasury and U.S. Government Agencies [Member]
Financial Assets:
Total fixed-maturity securities	40,527	1,939
Fair Value, Measurements, Recurring [Member] | (Level 1) [Member] | Corporate Bonds [Member]
Financial Assets:
Total fixed-maturity securities	106,109	73,519
Fair Value, Measurements, Recurring [Member] | (Level 1) [Member] | Exchange-Traded Debt [Member]
Financial Assets:
Total fixed-maturity securities	7,659	11,262
Fair Value, Measurements, Recurring [Member] | (Level 1) [Member] | Redeemable Preferred Stock [Member]
Financial Assets:
Total fixed-maturity securities		237
Fair Value, Measurements, Recurring [Member] | (Level 2) [Member]
Financial Assets:
Total fixed-maturity securities	83,189	79,291
Total available-for-sale securities	83,189	79,291
Total financial assets	83,189	79,291
Fair Value, Measurements, Recurring [Member] | (Level 2) [Member] | U.S. Treasury and U.S. Government Agencies [Member]
Financial Assets:
Total fixed-maturity securities	1,500
Fair Value, Measurements, Recurring [Member] | (Level 2) [Member] | Corporate Bonds [Member]
Financial Assets:
Total fixed-maturity securities	994	985
Fair Value, Measurements, Recurring [Member] | (Level 2) [Member] | State, Municipalities, and Political Subdivisions [Member]
Financial Assets:
Total fixed-maturity securities	$ 80,695	$ 78,306