Investments - Gross Unrealized Gains and Losses and Cost or Amortized Cost and Fair Value of Investments Classified as Available-for-Sale (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Investment [Line Items]
Totals, Amortized Cost	$ 445,117	$ 479,772
Totals, Gross Unrealized Gains	13,112	3,758
Totals, Gross Unrealized Losses	(336)	(4,800)
Totals, Fair Value	457,893	478,730
States and Political Subdivisions [Member]
Investment [Line Items]
Totals, Amortized Cost	220,385	231,848
Totals, Gross Unrealized Gains	9,852	3,515
Totals, Gross Unrealized Losses	(15)	(2,118)
Totals, Fair Value	230,222	233,245
Corporate Bonds [Member]
Investment [Line Items]
Totals, Amortized Cost	151,705	173,904
Totals, Gross Unrealized Gains	2,953	243
Totals, Gross Unrealized Losses	(39)	(933)
Totals, Fair Value	154,619	173,214
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Totals, Amortized Cost	11,904	12,835
Totals, Gross Unrealized Gains	6
Totals, Gross Unrealized Losses	(58)	(320)
Totals, Fair Value	11,852	12,515
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Investment [Line Items]
Totals, Amortized Cost	61,123	61,185
Totals, Gross Unrealized Gains	301
Totals, Gross Unrealized Losses	(224)	(1,429)
Totals, Fair Value	$ 61,200	$ 59,756