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, 2017	Dec. 31, 2016
Investment [Line Items]
Held-to-Maturity, Amortized Cost	$ 594,776	$ 562,434
Held-to-Maturity, Gross Unrealized Gains	10,782	9,902
Held-to-Maturity, Gross Unrealized Losses	(2,263)	(3,405)
Held-to-Maturity, Fair Value	603,295	568,931
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	429,088	394,875
Held-to-Maturity, Gross Unrealized Gains	8,599	7,622
Held-to-Maturity, Gross Unrealized Losses	(1,895)	(3,014)
Held-to-Maturity, Fair Value	435,792	399,483
Corporate Bonds [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	140,553	143,858
Held-to-Maturity, Gross Unrealized Gains	440	423
Held-to-Maturity, Gross Unrealized Losses	(256)	(265)
Held-to-Maturity, Fair Value	140,737	144,016
Commercial Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost		70
Held-to-Maturity, Fair Value		70
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	9,163	9,967
Held-to-Maturity, Gross Unrealized Gains	861	948
Held-to-Maturity, Fair Value	10,024	10,915
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	14,135	11,737
Held-to-Maturity, Gross Unrealized Gains	723	746
Held-to-Maturity, Gross Unrealized Losses	(62)	(67)
Held-to-Maturity, Fair Value	14,796	12,416
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	1,837	1,927
Held-to-Maturity, Gross Unrealized Gains	159	163
Held-to-Maturity, Gross Unrealized Losses	(50)	(59)
Held-to-Maturity, Fair Value	$ 1,946	$ 2,031