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, 2015	Dec. 31, 2014
Investment [Line Items]
Held-to-Maturity, Amortized Cost	$ 660,250	$ 639,631
Held-to-Maturity, Gross Unrealized Gains	20,139	25,404
Held-to-Maturity, Gross Unrealized Losses	(496)	(664)
Held-to-Maturity, Fair Value	679,893	664,371
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	417,804	385,623
Held-to-Maturity, Gross Unrealized Gains	16,307	20,100
Held-to-Maturity, Gross Unrealized Losses	(195)	(58)
Held-to-Maturity, Fair Value	433,916	405,665
Corporate Bonds [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	170,176	176,880
Held-to-Maturity, Gross Unrealized Gains	555	374
Held-to-Maturity, Gross Unrealized Losses	(241)	(520)
Held-to-Maturity, Fair Value	170,490	176,734
Commercial Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	43,396	46,662
Held-to-Maturity, Gross Unrealized Gains	586	1,867
Held-to-Maturity, Fair Value	43,982	48,529
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	14,094	16,972
Held-to-Maturity, Gross Unrealized Gains	1,393	1,702
Held-to-Maturity, Gross Unrealized Losses	(1)	(2)
Held-to-Maturity, Fair Value	15,486	18,672
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	12,365	10,697
Held-to-Maturity, Gross Unrealized Gains	1,074	1,097
Held-to-Maturity, Gross Unrealized Losses		(2)
Held-to-Maturity, Fair Value	13,439	11,792
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	2,415	2,797
Held-to-Maturity, Gross Unrealized Gains	224	264
Held-to-Maturity, Gross Unrealized Losses	(59)	(82)
Held-to-Maturity, Fair Value	$ 2,580	$ 2,979