Investments - Gross Unrealized Gains and Losses and Amortized Cost and Fair Value of Investments Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Investment [Line Items]
Held-to-Maturity, Amortized Cost	$ 601,314	$ 645,164
Held-to-Maturity, Gross Unrealized Gains	19,710	18,063
Held-to-Maturity, Gross Unrealized Losses	(243)	(951)
Held-to-Maturity, Fair Value	620,781	662,276
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	418,155	408,447
Held-to-Maturity, Gross Unrealized Gains	15,675	15,352
Held-to-Maturity, Gross Unrealized Losses	(96)	(45)
Held-to-Maturity, Fair Value	433,734	423,754
Corporate Bonds [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	158,042	171,224
Held-to-Maturity, Gross Unrealized Gains	1,698	159
Held-to-Maturity, Gross Unrealized Losses	(81)	(810)
Held-to-Maturity, Fair Value	159,659	170,573
Commercial Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	290	37,494
Held-to-Maturity, Gross Unrealized Gains		204
Held-to-Maturity, Gross Unrealized Losses		(15)
Held-to-Maturity, Fair Value	290	37,683
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	10,943	13,223
Held-to-Maturity, Gross Unrealized Gains	1,193	1,249
Held-to-Maturity, Gross Unrealized Losses		(1)
Held-to-Maturity, Fair Value	12,136	14,471
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	11,863	12,487
Held-to-Maturity, Gross Unrealized Gains	986	897
Held-to-Maturity, Gross Unrealized Losses	(1)	(4)
Held-to-Maturity, Fair Value	12,848	13,380
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-Maturity, Amortized Cost	2,021	2,289
Held-to-Maturity, Gross Unrealized Gains	158	202
Held-to-Maturity, Gross Unrealized Losses	(65)	(76)
Held-to-Maturity, Fair Value	$ 2,114	$ 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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