Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Marketable Securities [Line Items]
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	$ 18,285	$ 48,682
Held-to-Maturity, Gross Unrealized Losses, Less Than 12 Months	72	99
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	11,613	278,163
Held-to-Maturity, Gross Unrealized Losses, 12 Months or Greater	65	2,867
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Total	29,898	326,845
Held-to-Maturity, Gross Unrealized Losses, Total	137	2,966
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	18,391	75,208
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	39	360
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	28,069	218,832
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	106	4,440
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	46,460	294,040
Available-for Sale, Gross Unrealized Losses, Total	145	4,800
Fair Value of Investments with Unrealized Losses, Less Than 12 months	36,676	123,890
Gross Unrealized Loss, Less Than 12 Months	111	459
Fair Value of Investments with Unrealized Losses, 12 Months or Greater	39,682	496,995
Gross Unrealized Losses, 12 Months or Greater	171	7,307
Fair Value of Investments with Unrealized Losses, Total	76,358	620,885
Gross Unrealized Losses, Total	282	7,766
States and Political Subdivisions [Member]
Marketable Securities [Line Items]
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	4,277	28,369
Held-to-Maturity, Gross Unrealized Losses, Less Than 12 Months	3	59
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, 12 Months or Greater		180,550
Held-to-Maturity, Gross Unrealized Losses, 12 Months or Greater		2,025
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Total	4,277	208,919
Held-to-Maturity, Gross Unrealized Losses, Total	3	2,084
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	4,189	16,109
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	2	81
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater		76,255
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater		2,037
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	4,189	92,364
Available-for Sale, Gross Unrealized Losses, Total	2	2,118
Corporate Bonds [Member]
Marketable Securities [Line Items]
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	13,241	17,448
Held-to-Maturity, Gross Unrealized Losses, Less Than 12 Months	67	36
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	8,208	48,315
Held-to-Maturity, Gross Unrealized Losses, 12 Months or Greater	41	419
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Total	21,449	65,763
Held-to-Maturity, Gross Unrealized Losses, Total	108	455
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months		59,099
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months		279
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	7,093	70,306
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	7	654
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	7,093	129,405
Available-for Sale, Gross Unrealized Losses, Total	7	933
U.S. Agency-Based Mortgage-Backed Securities [Member]
Marketable Securities [Line Items]
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	404
Held-to-Maturity, Gross Unrealized Losses, Less Than 12 Months	1
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, 12 Months or Greater		2,287
Held-to-Maturity, Gross Unrealized Losses, 12 Months or Greater		80
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Total	404	2,287
Held-to-Maturity, Gross Unrealized Losses, Total	1	80
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	13,194
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	35
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	3,928	12,515
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	20	320
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	17,122	12,515
Available-for Sale, Gross Unrealized Losses, Total	55	320
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Marketable Securities [Line Items]
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Less Than 12 Months		2,865
Held-to-Maturity, Gross Unrealized Losses, Less Than 12 Months		4
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	3,236	46,486
Held-to-Maturity, Gross Unrealized Losses, 12 Months or Greater	21	335
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Total	3,236	49,351
Held-to-Maturity, Gross Unrealized Losses, Total	21	339
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	1,008
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	2
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	17,048	59,756
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	79	1,429
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	18,056	59,756
Available-for Sale, Gross Unrealized Losses, Total	81	1,429
Asset-Backed Securities [Member]
Marketable Securities [Line Items]
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	363
Held-to-Maturity, Gross Unrealized Losses, Less Than 12 Months	1
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	169	525
Held-to-Maturity, Gross Unrealized Losses, 12 Months or Greater	3	8
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Total	532	525
Held-to-Maturity, Gross Unrealized Losses, Total	$ 4	$ 8

Continuous unrealized loss position greater than twelve months aggregate losses.

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Continuous unrealized loss position greater than twelve months fair value.

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Continuous unrealized loss position less than twelve months aggregate losses.

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Continuous unrealized loss position less than twelve months fair value.

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Securities continuous unrealized loss position aggregate losses.

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Securities continuous unrealized loss position fair value.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position without allowance for credit loss.

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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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

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