Investments - Gross Unrealized Gains and Losses and Amortized Cost and Fair Value of Investments Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 598,658	$ 562,434
Held-to-Maturity, Gross Unrealized Gains	12,370	9,902
Held-to-Maturity, Gross Unrealized Losses	(877)	(3,405)
Held-to-Maturity, Fair Value	610,151	568,931
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	430,470	394,875
Held-to-Maturity, Gross Unrealized Gains	10,348	7,622
Held-to-Maturity, Gross Unrealized Losses	(573)	(3,014)
Held-to-Maturity, Fair Value	440,245	399,483
Corporate Bonds [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	121,942	143,858
Held-to-Maturity, Gross Unrealized Gains	498	423
Held-to-Maturity, Gross Unrealized Losses	(192)	(265)
Held-to-Maturity, Fair Value	122,248	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	10,928	9,967
Held-to-Maturity, Gross Unrealized Gains	750	948
Held-to-Maturity, Gross Unrealized Losses	(2)
Held-to-Maturity, Fair Value	11,676	10,915
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	33,672	11,737
Held-to-Maturity, Gross Unrealized Gains	642	746
Held-to-Maturity, Gross Unrealized Losses	(94)	(67)
Held-to-Maturity, Fair Value	34,220	12,416
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost, Totals	1,646	1,927
Held-to-Maturity, Gross Unrealized Gains	132	163
Held-to-Maturity, Gross Unrealized Losses	(16)	(59)
Held-to-Maturity, Fair Value	$ 1,762	$ 2,031