Investments - Gross Unrealized Gains and Losses and Amortized Cost and Fair Value of Investments Classified as Held-to-Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Investment [Line Items]
Held-to-Maturity, Amortized Cost	$ 639,631us-gaap_HeldToMaturitySecurities	$ 536,583us-gaap_HeldToMaturitySecurities
Held-to-Maturity, Gross Unrealized Gains	25,404	25,934
Held-to-Maturity, Gross Unrealized Losses	(664)	(1,338)
Held-to-Maturity, Fair Value	664,371	561,179
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	385,623	381,674
Held-to-Maturity, Gross Unrealized Gains	20,100	18,634
Held-to-Maturity, Gross Unrealized Losses	(58)	(1,153)
Held-to-Maturity, Fair Value	405,665	399,155
Corporate Bonds [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	176,880	67,423
Held-to-Maturity, Gross Unrealized Gains	374	861
Held-to-Maturity, Gross Unrealized Losses	(520)	(41)
Held-to-Maturity, Fair Value	176,734	68,243
Commercial Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	46,662	50,813
Held-to-Maturity, Gross Unrealized Gains	1,867	3,431
Held-to-Maturity, Fair Value	48,529	54,244
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	16,972	21,775
Held-to-Maturity, Gross Unrealized Gains	1,702	1,790
Held-to-Maturity, Gross Unrealized Losses	(2)
Held-to-Maturity, Fair Value	18,672	23,565
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	10,697	11,514
Held-to-Maturity, Gross Unrealized Gains	1,097	1,002
Held-to-Maturity, Gross Unrealized Losses	(2)
Held-to-Maturity, Fair Value	11,792	12,516
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	2,797	3,384
Held-to-Maturity, Gross Unrealized Gains	264	216
Held-to-Maturity, Gross Unrealized Losses	(82)	(144)
Held-to-Maturity, Fair Value	$ 2,979	$ 3,456

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity.

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Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity.

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Fair value of investments in debt securities classified as held-to-maturity.

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