Investments - Gross Unrealized Gains and Losses and Amortized Cost and Fair Value of Investments Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 629,668	$ 562,434
Held-to-Maturity, Gross Unrealized Gains	11,016	9,902
Held-to-Maturity, Gross Unrealized Losses	(1,375)	(3,405)
Held-to-Maturity, Fair Value	639,309	568,931
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	460,428	394,875
Held-to-Maturity, Gross Unrealized Gains	9,628	7,622
Held-to-Maturity, Gross Unrealized Losses	(955)	(3,014)
Held-to-Maturity, Fair Value	469,101	399,483
Corporate Bonds [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	100,024	143,858
Held-to-Maturity, Gross Unrealized Gains	190	423
Held-to-Maturity, Gross Unrealized Losses	(167)	(265)
Held-to-Maturity, Fair Value	100,047	144,016
Commercial Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals		70
Held-to-Maturity, Fair Value		70
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	10,260	9,967
Held-to-Maturity, Gross Unrealized Gains	625	948
Held-to-Maturity, Gross Unrealized Losses	(40)
Held-to-Maturity, Fair Value	10,845	10,915
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	57,657	11,737
Held-to-Maturity, Gross Unrealized Gains	548	746
Held-to-Maturity, Gross Unrealized Losses	(198)	(67)
Held-to-Maturity, Fair Value	58,007	12,416
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	1,299	1,927
Held-to-Maturity, Gross Unrealized Gains	25	163
Held-to-Maturity, Gross Unrealized Losses	(15)	(59)
Held-to-Maturity, Fair Value	$ 1,309	$ 2,031