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, 2017	Dec. 31, 2016
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 599,096	$ 562,434
Held-to-Maturity, Gross Unrealized Gains	12,498	9,902
Held-to-Maturity, Gross Unrealized Losses	(1,043)	(3,405)
Held-to-Maturity, Fair Value	610,551	568,931
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	430,921	394,875
Held-to-Maturity, Gross Unrealized Gains	10,435	7,622
Held-to-Maturity, Gross Unrealized Losses	(777)	(3,014)
Held-to-Maturity, Fair Value	440,579	399,483
Corporate Bonds [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	129,143	143,858
Held-to-Maturity, Gross Unrealized Gains	438	423
Held-to-Maturity, Gross Unrealized Losses	(176)	(265)
Held-to-Maturity, Fair Value	129,405	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	9,171	9,967
Held-to-Maturity, Gross Unrealized Gains	797	948
Held-to-Maturity, Gross Unrealized Losses	(2)
Held-to-Maturity, Fair Value	9,966	10,915
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	28,113	11,737
Held-to-Maturity, Gross Unrealized Gains	689	746
Held-to-Maturity, Gross Unrealized Losses	(72)	(67)
Held-to-Maturity, Fair Value	28,730	12,416
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	1,748	1,927
Held-to-Maturity, Gross Unrealized Gains	139	163
Held-to-Maturity, Gross Unrealized Losses	(16)	(59)
Held-to-Maturity, Fair Value	$ 1,871	$ 2,031