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, 2019	Dec. 31, 2018
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 604,056	$ 613,878
Held-to-Maturity, Gross Unrealized Gains	11,760	5,860
Held-to-Maturity, Gross Unrealized Losses	(599)	(2,966)
Held-to-Maturity, Fair Value	615,217	616,772
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	431,247	445,922
Held-to-Maturity, Gross Unrealized Gains	10,468	5,109
Held-to-Maturity, Gross Unrealized Losses	(242)	(2,084)
Held-to-Maturity, Fair Value	441,473	448,947
Corporate Bonds [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	102,774	91,762
Held-to-Maturity, Gross Unrealized Gains	520	62
Held-to-Maturity, Gross Unrealized Losses	(155)	(455)
Held-to-Maturity, Fair Value	103,139	91,369
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	9,824	8,102
Held-to-Maturity, Gross Unrealized Gains	396	327
Held-to-Maturity, Gross Unrealized Losses	(56)	(80)
Held-to-Maturity, Fair Value	10,164	8,349
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	59,210	67,042
Held-to-Maturity, Gross Unrealized Gains	352	340
Held-to-Maturity, Gross Unrealized Losses	(140)	(339)
Held-to-Maturity, Fair Value	59,422	67,043
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	1,001	1,050
Held-to-Maturity, Gross Unrealized Gains	24	22
Held-to-Maturity, Gross Unrealized Losses	(6)	(8)
Held-to-Maturity, Fair Value	$ 1,019	$ 1,064