Fair Value Measurements - Available-for-Sale Securities Measured at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Financial Assets:
Total fixed-maturity securities	$ 166,248	$ 125,009
Equity securities	53,035	48,237
Total available-for-sale securities	219,283	173,246
Fair Value, Measurements, Recurring [Member]
Financial Assets:
Cash and cash equivalents	280,531	267,738
Total fixed-maturity securities	166,248	125,009
Equity securities	53,035	48,237
Total available-for-sale securities	219,283	173,246
Total financial assets	499,814	440,984
Fair Value, Measurements, Recurring [Member] | U.S. Treasury and U.S. Government Agencies [Member]
Financial Assets:
Total fixed-maturity securities	1,939	113
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]
Financial Assets:
Total fixed-maturity securities	74,504	37,819
Fair Value, Measurements, Recurring [Member] | State, Municipalities, and Political Subdivisions [Member]
Financial Assets:
Total fixed-maturity securities	78,306	77,324
Fair Value, Measurements, Recurring [Member] | Exchange-Traded Debt [Member]
Financial Assets:
Total fixed-maturity securities	11,262	9,429
Fair Value, Measurements, Recurring [Member] | Redeemable Preferred Stock [Member]
Financial Assets:
Total fixed-maturity securities	237	324
Fair Value, Measurements, Recurring [Member] | (Level 1) [Member]
Financial Assets:
Cash and cash equivalents	280,531	267,738
Total fixed-maturity securities	86,957	46,702
Equity securities	53,035	48,237
Total available-for-sale securities	139,992	94,939
Total financial assets	420,523	362,677
Fair Value, Measurements, Recurring [Member] | (Level 1) [Member] | U.S. Treasury and U.S. Government Agencies [Member]
Financial Assets:
Total fixed-maturity securities	1,939	113
Fair Value, Measurements, Recurring [Member] | (Level 1) [Member] | Corporate Bonds [Member]
Financial Assets:
Total fixed-maturity securities	73,519	36,836
Fair Value, Measurements, Recurring [Member] | (Level 1) [Member] | Exchange-Traded Debt [Member]
Financial Assets:
Total fixed-maturity securities	11,262	9,429
Fair Value, Measurements, Recurring [Member] | (Level 1) [Member] | Redeemable Preferred Stock [Member]
Financial Assets:
Total fixed-maturity securities	237	324
Fair Value, Measurements, Recurring [Member] | (Level 2) [Member]
Financial Assets:
Total fixed-maturity securities	79,291	78,307
Total available-for-sale securities	79,291	78,307
Total financial assets	79,291	78,307
Fair Value, Measurements, Recurring [Member] | (Level 2) [Member] | Corporate Bonds [Member]
Financial Assets:
Total fixed-maturity securities	985	983
Fair Value, Measurements, Recurring [Member] | (Level 2) [Member] | State, Municipalities, and Political Subdivisions [Member]
Financial Assets:
Total fixed-maturity securities	$ 78,306	$ 77,324