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, 2015	Dec. 31, 2014
Investment [Line Items]
Held-to-Maturity, Amortized Cost	$ 698,513	$ 639,631
Held-to-Maturity, Gross Unrealized Gains	19,759	25,404
Held-to-Maturity, Gross Unrealized Losses	(1,416)	(664)
Held-to-Maturity, Fair Value	716,856	664,371
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	439,985	385,623
Held-to-Maturity, Gross Unrealized Gains	15,536	20,100
Held-to-Maturity, Gross Unrealized Losses	(961)	(58)
Held-to-Maturity, Fair Value	454,560	405,665
Corporate Bonds [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	183,246	176,880
Held-to-Maturity, Gross Unrealized Gains	425	374
Held-to-Maturity, Gross Unrealized Losses	(384)	(520)
Held-to-Maturity, Fair Value	183,287	176,734
Commercial Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	44,907	46,662
Held-to-Maturity, Gross Unrealized Gains	1,001	1,867
Held-to-Maturity, Fair Value	45,908	48,529
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	14,974	16,972
Held-to-Maturity, Gross Unrealized Gains	1,516	1,702
Held-to-Maturity, Gross Unrealized Losses	(2)	(2)
Held-to-Maturity, Fair Value	16,488	18,672
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	12,869	10,697
Held-to-Maturity, Gross Unrealized Gains	1,045	1,097
Held-to-Maturity, Gross Unrealized Losses		(2)
Held-to-Maturity, Fair Value	13,914	11,792
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	2,532	2,797
Held-to-Maturity, Gross Unrealized Gains	236	264
Held-to-Maturity, Gross Unrealized Losses	(69)	(82)
Held-to-Maturity, Fair Value	$ 2,699	$ 2,979