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, 2019	Dec. 31, 2018
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 604,161	$ 613,878
Held-to-Maturity, Gross Unrealized Gains	22,579	5,860
Held-to-Maturity, Gross Unrealized Losses	(137)	(2,966)
Held-to-Maturity, Fair Value	626,603	616,772
States and Political Subdivisions [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	438,055	445,922
Held-to-Maturity, Gross Unrealized Gains	19,978	5,109
Held-to-Maturity, Gross Unrealized Losses	(3)	(2,084)
Held-to-Maturity, Fair Value	458,030	448,947
Corporate Bonds [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	115,982	91,762
Held-to-Maturity, Gross Unrealized Gains	1,587	62
Held-to-Maturity, Gross Unrealized Losses	(108)	(455)
Held-to-Maturity, Fair Value	117,461	91,369
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	11,910	8,102
Held-to-Maturity, Gross Unrealized Gains	607	327
Held-to-Maturity, Gross Unrealized Losses	(1)	(80)
Held-to-Maturity, Fair Value	12,516	8,349
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	37,513	67,042
Held-to-Maturity, Gross Unrealized Gains	406	340
Held-to-Maturity, Gross Unrealized Losses	(21)	(339)
Held-to-Maturity, Fair Value	37,898	67,043
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	701	1,050
Held-to-Maturity, Gross Unrealized Gains	1	22
Held-to-Maturity, Gross Unrealized Losses	(4)	(8)
Held-to-Maturity, Fair Value	$ 698	$ 1,064

Amount of investment in debt security measured at amortized cost (held-to-maturity).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27232-111563

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Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27232-111563

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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
-Paragraph 5
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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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