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, 2015	Dec. 31, 2014
Investment [Line Items]
Held-to-Maturity, Amortized Cost	$ 645,164	$ 639,631
Held-to-Maturity, Gross Unrealized Gains	18,063	25,404
Held-to-Maturity, Gross Unrealized Losses	(951)	(664)
Held-to-Maturity, Fair Value	662,276	664,371
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	408,447	385,623
Held-to-Maturity, Gross Unrealized Gains	15,352	20,100
Held-to-Maturity, Gross Unrealized Losses	(45)	(58)
Held-to-Maturity, Fair Value	423,754	405,665
Corporate Bonds [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	171,224	176,880
Held-to-Maturity, Gross Unrealized Gains	159	374
Held-to-Maturity, Gross Unrealized Losses	(810)	(520)
Held-to-Maturity, Fair Value	170,573	176,734
Commercial Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	37,494	46,662
Held-to-Maturity, Gross Unrealized Gains	204	1,867
Held-to-Maturity, Gross Unrealized Losses	(15)
Held-to-Maturity, Fair Value	37,683	48,529
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	13,223	16,972
Held-to-Maturity, Gross Unrealized Gains	1,249	1,702
Held-to-Maturity, Gross Unrealized Losses	(1)	(2)
Held-to-Maturity, Fair Value	14,471	18,672
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	12,487	10,697
Held-to-Maturity, Gross Unrealized Gains	897	1,097
Held-to-Maturity, Gross Unrealized Losses	(4)	(2)
Held-to-Maturity, Fair Value	13,380	11,792
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	2,289	2,797
Held-to-Maturity, Gross Unrealized Gains	202	264
Held-to-Maturity, Gross Unrealized Losses	(76)	(82)
Held-to-Maturity, Fair Value	$ 2,415	$ 2,979

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51813488&loc=d3e22054-111558

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.6)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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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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- Definition

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

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