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, 2019	Dec. 31, 2018
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 610,950	$ 613,878
Held-to-Maturity, Gross Unrealized Gains	18,354	5,860
Held-to-Maturity, Gross Unrealized Losses	(97)	(2,966)
Held-to-Maturity, Fair Value	629,207	616,772
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	439,103	445,922
Held-to-Maturity, Gross Unrealized Gains	15,966	5,109
Held-to-Maturity, Gross Unrealized Losses	(36)	(2,084)
Held-to-Maturity, Fair Value	455,033	448,947
Corporate Bonds [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	108,756	91,762
Held-to-Maturity, Gross Unrealized Gains	1,425	62
Held-to-Maturity, Gross Unrealized Losses	(17)	(455)
Held-to-Maturity, Fair Value	110,164	91,369
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	12,465	8,102
Held-to-Maturity, Gross Unrealized Gains	523	327
Held-to-Maturity, Gross Unrealized Losses	(12)	(80)
Held-to-Maturity, Fair Value	12,976	8,349
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	49,677	67,042
Held-to-Maturity, Gross Unrealized Gains	430	340
Held-to-Maturity, Gross Unrealized Losses	(29)	(339)
Held-to-Maturity, Fair Value	50,078	67,043
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	949	1,050
Held-to-Maturity, Gross Unrealized Gains	10	22
Held-to-Maturity, Gross Unrealized Losses	(3)	(8)
Held-to-Maturity, Fair Value	$ 956	$ 1,064