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	Mar. 31, 2015	Dec. 31, 2014
Investment [Line Items]
Held-to-Maturity, Amortized Cost	$ 686,618us-gaap_HeldToMaturitySecurities	$ 639,631us-gaap_HeldToMaturitySecurities
Held-to-Maturity, Gross Unrealized Gains	24,979	25,404
Held-to-Maturity, Gross Unrealized Losses	(333)	(664)
Held-to-Maturity, Fair Value	711,264	664,371
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	417,361	385,623
Held-to-Maturity, Gross Unrealized Gains	19,559	20,100
Held-to-Maturity, Gross Unrealized Losses	(46)	(58)
Held-to-Maturity, Fair Value	436,874	405,665
Corporate Bonds [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	191,594	176,880
Held-to-Maturity, Gross Unrealized Gains	812	374
Held-to-Maturity, Gross Unrealized Losses	(207)	(520)
Held-to-Maturity, Fair Value	192,199	176,734
Commercial Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	46,094	46,662
Held-to-Maturity, Gross Unrealized Gains	1,495	1,867
Held-to-Maturity, Fair Value	47,589	48,529
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	15,953	16,972
Held-to-Maturity, Gross Unrealized Gains	1,691	1,702
Held-to-Maturity, Gross Unrealized Losses	(2)	(2)
Held-to-Maturity, Fair Value	17,642	18,672
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	12,932	10,697
Held-to-Maturity, Gross Unrealized Gains	1,176	1,097
Held-to-Maturity, Gross Unrealized Losses		(2)
Held-to-Maturity, Fair Value	14,108	11,792
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	2,684	2,797
Held-to-Maturity, Gross Unrealized Gains	246	264
Held-to-Maturity, Gross Unrealized Losses	(78)	(82)
Held-to-Maturity, Fair Value	$ 2,852	$ 2,979

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

+ References

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-Name Accounting Standards Codification

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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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