Investments - Gross Unrealized Gains and Losses and Amortized Cost and Fair Value of Investments Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Investment [Line Items]
Held-to-Maturity, Amortized Cost	$ 562,434	$ 645,164
Held-to-Maturity, Gross Unrealized Gains	9,902	18,063
Held-to-Maturity, Gross Unrealized Losses	(3,405)	(951)
Held-to-Maturity, Fair Value	568,931	662,276
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	394,875	408,447
Held-to-Maturity, Gross Unrealized Gains	7,622	15,352
Held-to-Maturity, Gross Unrealized Losses	(3,014)	(45)
Held-to-Maturity, Fair Value	399,483	423,754
Corporate Bonds [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	143,858	171,224
Held-to-Maturity, Gross Unrealized Gains	423	159
Held-to-Maturity, Gross Unrealized Losses	(265)	(810)
Held-to-Maturity, Fair Value	144,016	170,573
Commercial Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	70	37,494
Held-to-Maturity, Gross Unrealized Gains		204
Held-to-Maturity, Gross Unrealized Losses		(15)
Held-to-Maturity, Fair Value	70	37,683
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	9,967	13,223
Held-to-Maturity, Gross Unrealized Gains	948	1,249
Held-to-Maturity, Gross Unrealized Losses		(1)
Held-to-Maturity, Fair Value	10,915	14,471
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	11,737	12,487
Held-to-Maturity, Gross Unrealized Gains	746	897
Held-to-Maturity, Gross Unrealized Losses	(67)	(4)
Held-to-Maturity, Fair Value	12,416	13,380
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	1,927	2,289
Held-to-Maturity, Gross Unrealized Gains	163	202
Held-to-Maturity, Gross Unrealized Losses	(59)	(76)
Held-to-Maturity, Fair Value	$ 2,031	$ 2,415