Investments - Gross Unrealized Gains and Losses and Amortized Cost and Fair Value of Investments Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 619,400	$ 629,668
Held-to-Maturity, Gross Unrealized Gains	6,055	11,016
Held-to-Maturity, Gross Unrealized Losses	(4,519)	(1,375)
Held-to-Maturity, Fair Value	620,936	639,309
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	450,553	460,428
Held-to-Maturity, Gross Unrealized Gains	5,041	9,628
Held-to-Maturity, Gross Unrealized Losses	(3,248)	(955)
Held-to-Maturity, Fair Value	452,346	469,101
Corporate Bonds [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	99,368	100,024
Held-to-Maturity, Gross Unrealized Gains	29	190
Held-to-Maturity, Gross Unrealized Losses	(650)	(167)
Held-to-Maturity, Fair Value	98,747	100,047
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	9,400	10,260
Held-to-Maturity, Gross Unrealized Gains	503	625
Held-to-Maturity, Gross Unrealized Losses	(72)	(40)
Held-to-Maturity, Fair Value	9,831	10,845
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	58,857	57,657
Held-to-Maturity, Gross Unrealized Gains	456	548
Held-to-Maturity, Gross Unrealized Losses	(542)	(198)
Held-to-Maturity, Fair Value	58,771	58,007
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	1,222	1,299
Held-to-Maturity, Gross Unrealized Gains	26	25
Held-to-Maturity, Gross Unrealized Losses	(7)	(15)
Held-to-Maturity, Fair Value	$ 1,241	$ 1,309