Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	$ 48,682	$ 218,074
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	99	972
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	278,163	28,931
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	2,867	403
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	326,845	247,005
Held-to-maturity securities, Gross Unrealized Losses, Total	2,966	1,375
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	75,208	135,576
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	360	520
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	218,832	76,085
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	4,440	2,194
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	294,040	211,661
Available-for Sale, Gross Unrealized Losses, Total	4,800	2,714
Fair Value of Investments with Unrealized Losses, Less Than 12 months	123,890	353,650
Gross Unrealized Loss, Less Than 12 Months	459	1,492
Fair Value of Investments with Unrealized Losses, 12 Months or Greater	496,995	105,016
Gross Unrealized Losses, 12 Months or Greater	7,307	2,597
Fair Value of Investments with Unrealized Losses, Total	620,885	458,666
Gross Unrealized Losses, Total	7,766	4,089
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	110,698
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	59	654
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	180,550	19,895
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	2,025	301
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	208,919	130,593
Held-to-maturity securities, Gross Unrealized Losses, Total	2,084	955
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	16,109	23,365
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	81	86
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	76,255	19,153
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	2,037	491
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	92,364	42,518
Available-for Sale, Gross Unrealized Losses, Total	2,118	577
Corporate Bonds [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	17,448	56,425
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	36	156
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	48,315	4,121
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	419	11
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	65,763	60,546
Held-to-maturity securities, Gross Unrealized Losses, Total	455	167
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	59,099	82,795
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	279	171
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	70,306	5,888
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	654	41
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	129,405	88,683
Available-for Sale, Gross Unrealized Losses, Total	933	212
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months		2,798
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months		40
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	2,287
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	80
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	2,287	2,798
Held-to-maturity securities, Gross Unrealized Losses, Total	80	40
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months		14,686
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months		59
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	12,515	3,328
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	320	740
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	12,515	18,014
Available-for Sale, Gross Unrealized Losses, Total	320	799
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	48,153
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	4	122
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	46,486	3,948
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	335	76
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	49,351	52,101
Held-to-maturity securities, Gross Unrealized Losses, Total	339	198
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months		14,730
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months		204
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	59,756	47,716
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	1,429	922
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	59,756	62,446
Available-for Sale, Gross Unrealized Losses, Total	1,429	1,126
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	525	967
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	8	15
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	525	967
Held-to-maturity securities, Gross Unrealized Losses, Total	$ 8	$ 15