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, 2016	Dec. 31, 2015
Investment [Line Items]
Held-to-Maturity, Amortized Cost	$ 620,165	$ 645,164
Held-to-Maturity, Gross Unrealized Gains	23,362	18,063
Held-to-Maturity, Gross Unrealized Losses	(203)	(951)
Held-to-Maturity, Fair Value	643,324	662,276
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	423,631	408,447
Held-to-Maturity, Gross Unrealized Gains	19,066	15,352
Held-to-Maturity, Gross Unrealized Losses	(3)	(45)
Held-to-Maturity, Fair Value	442,694	423,754
Corporate Bonds [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	163,648	171,224
Held-to-Maturity, Gross Unrealized Gains	1,739	159
Held-to-Maturity, Gross Unrealized Losses	(117)	(810)
Held-to-Maturity, Fair Value	165,270	170,573
Commercial Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	6,681	37,494
Held-to-Maturity, Gross Unrealized Gains	37	204
Held-to-Maturity, Gross Unrealized Losses	(6)	(15)
Held-to-Maturity, Fair Value	6,712	37,683
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	11,849	13,223
Held-to-Maturity, Gross Unrealized Gains	1,280	1,249
Held-to-Maturity, Gross Unrealized Losses	(1)	(1)
Held-to-Maturity, Fair Value	13,128	14,471
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	12,261	12,487
Held-to-Maturity, Gross Unrealized Gains	1,057	897
Held-to-Maturity, Gross Unrealized Losses		(4)
Held-to-Maturity, Fair Value	13,318	13,380
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	2,095	2,289
Held-to-Maturity, Gross Unrealized Gains	183	202
Held-to-Maturity, Gross Unrealized Losses	(76)	(76)
Held-to-Maturity, Fair Value	$ 2,202	$ 2,415

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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=73527792&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=65884222&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=73527792&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.

+ References

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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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