Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	$ 3,728	$ 48,682
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	5	99
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	159,421	278,163
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	594	2,867
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	163,149	326,845
Held-to-maturity securities, Gross Unrealized Losses, Total	599	2,966
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	7,691	75,208
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	8	360
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	172,166	218,832
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	1,693	4,440
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	179,857	294,040
Available-for Sale, Gross Unrealized Losses, Total	1,701	4,800
Fair Value of Investments with Unrealized Losses, Less Than 12 months	11,419	123,890
Gross Unrealized Loss, Less Than 12 Months	13	459
Fair Value of Investments with Unrealized Losses, 12 Months or Greater	331,587	496,995
Gross Unrealized Losses, 12 Months or Greater	2,287	7,307
Fair Value of Investments with Unrealized Losses, Total	343,006	620,885
Gross Unrealized Losses, Total	2,300	7,766
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months		28,369
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months		59
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	78,248	180,550
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	242	2,025
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	78,248	208,919
Held-to-maturity securities, Gross Unrealized Losses, Total	242	2,084
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	1,957	16,109
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	1	81
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	27,768	76,255
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	394	2,037
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	29,725	92,364
Available-for Sale, Gross Unrealized Losses, Total	395	2,118
Corporate Bonds [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	3,728	17,448
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	5	36
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	37,828	48,315
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	150	419
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	41,556	65,763
Held-to-maturity securities, Gross Unrealized Losses, Total	155	455
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	5,734	59,099
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	7	279
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	75,651	70,306
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	210	654
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	81,385	129,405
Available-for Sale, Gross Unrealized Losses, Total	217	933
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	2,248	2,287
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	56	80
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	2,248	2,287
Held-to-maturity securities, Gross Unrealized Losses, Total	56	80
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	12,281	12,515
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	180	320
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	12,281	12,515
Available-for Sale, Gross Unrealized Losses, Total	180	320
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months		2,865
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months		4
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	40,587	46,486
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	140	335
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	40,587	49,351
Held-to-maturity securities, Gross Unrealized Losses, Total	140	339
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	56,466	59,756
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	909	1,429
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	56,466	59,756
Available-for Sale, Gross Unrealized Losses, Total	909	1,429
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	510	525
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	6	8
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	510	525
Held-to-maturity securities, Gross Unrealized Losses, Total	$ 6	$ 8