Investments - Gross Unrealized Gains and Losses and Amortized Cost and Fair Value of Investments Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 602,830	$ 629,668
Held-to-Maturity, Gross Unrealized Gains	3,770	11,016
Held-to-Maturity, Gross Unrealized Losses	(6,715)	(1,375)
Held-to-Maturity, Fair Value	599,885	639,309
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	430,408	460,428
Held-to-Maturity, Gross Unrealized Gains	3,053	9,628
Held-to-Maturity, Gross Unrealized Losses	(4,773)	(955)
Held-to-Maturity, Fair Value	428,688	469,101
Corporate Bonds [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	95,731	100,024
Held-to-Maturity, Gross Unrealized Gains	9	190
Held-to-Maturity, Gross Unrealized Losses	(879)	(167)
Held-to-Maturity, Fair Value	94,861	100,047
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	8,535	10,260
Held-to-Maturity, Gross Unrealized Gains	359	625
Held-to-Maturity, Gross Unrealized Losses	(87)	(40)
Held-to-Maturity, Fair Value	8,807	10,845
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	67,052	57,657
Held-to-Maturity, Gross Unrealized Gains	325	548
Held-to-Maturity, Gross Unrealized Losses	(966)	(198)
Held-to-Maturity, Fair Value	66,411	58,007
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	1,104	1,299
Held-to-Maturity, Gross Unrealized Gains	24	25
Held-to-Maturity, Gross Unrealized Losses	(10)	(15)
Held-to-Maturity, Fair Value	$ 1,118	$ 1,309