Investments - Gross Unrealized Gains and Losses and Amortized Cost and Fair Value of Investments Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 605,877	$ 629,668
Held-to-Maturity, Gross Unrealized Gains	5,582	11,016
Held-to-Maturity, Gross Unrealized Losses	(4,387)	(1,375)
Held-to-Maturity, Fair Value	607,072	639,309
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	432,914	460,428
Held-to-Maturity, Gross Unrealized Gains	4,714	9,628
Held-to-Maturity, Gross Unrealized Losses	(2,891)	(955)
Held-to-Maturity, Fair Value	434,737	469,101
Corporate Bonds [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	98,702	100,024
Held-to-Maturity, Gross Unrealized Gains	38	190
Held-to-Maturity, Gross Unrealized Losses	(724)	(167)
Held-to-Maturity, Fair Value	98,016	100,047
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	9,038	10,260
Held-to-Maturity, Gross Unrealized Gains	420	625
Held-to-Maturity, Gross Unrealized Losses	(71)	(40)
Held-to-Maturity, Fair Value	9,387	10,845
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	64,063	57,657
Held-to-Maturity, Gross Unrealized Gains	385	548
Held-to-Maturity, Gross Unrealized Losses	(691)	(198)
Held-to-Maturity, Fair Value	63,757	58,007
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	1,160	1,299
Held-to-Maturity, Gross Unrealized Gains	25	25
Held-to-Maturity, Gross Unrealized Losses	(10)	(15)
Held-to-Maturity, Fair Value	$ 1,175	$ 1,309