Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	$ 353,557	$ 218,074
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	3,955	972
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	21,287	28,931
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	564	403
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	374,844	247,005
Held-to-maturity securities, Gross Unrealized Losses, Total	4,519	1,375
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	267,995	135,576
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	3,727	520
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	51,785	76,085
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	2,777	2,194
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	319,780	211,661
Available-for Sale, Gross Unrealized Losses, Total	6,504	2,714
Fair Value of Investments with Unrealized Losses, Less Than 12 months	621,552	353,650
Gross Unrealized Loss, Less Than 12 Months	7,682	1,492
Fair Value of Investments with Unrealized Losses, 12 Months or Greater	73,072	105,016
Gross Unrealized Losses, 12 Months or Greater	3,341	2,597
Fair Value of Investments with Unrealized Losses, Total	694,624	458,666
Gross Unrealized Losses, Total	11,023	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	212,693	110,698
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	2,810	654
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	14,513	19,895
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	438	301
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	227,206	130,593
Held-to-maturity securities, Gross Unrealized Losses, Total	3,248	955
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	89,896	23,365
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	1,620	86
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	8,892	19,153
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	790	491
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	98,788	42,518
Available-for Sale, Gross Unrealized Losses, Total	2,410	577
Corporate Bonds [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	89,227	56,425
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	629	156
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	2,259	4,121
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	21	11
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	91,486	60,546
Held-to-maturity securities, Gross Unrealized Losses, Total	650	167
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	138,600	82,795
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	943	171
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater		5,888
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater		41
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	138,600	88,683
Available-for Sale, Gross Unrealized Losses, Total	943	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	2,613	2,798
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	72	40
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	2,613	2,798
Held-to-maturity securities, Gross Unrealized Losses, Total	72	40
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	14,477	14,686
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	451	59
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	2,508	3,328
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	824	740
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	16,985	18,014
Available-for Sale, Gross Unrealized Losses, Total	1,275	799
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	535
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	5
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	47	967
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	2	15
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	582	967
Held-to-maturity securities, Gross Unrealized Losses, Total	7	15
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	48,489	48,153
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	439	122
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	4,468	3,948
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	103	76
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	52,957	52,101
Held-to-maturity securities, Gross Unrealized Losses, Total	542	198
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	25,022	14,730
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	713	204
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	40,385	47,716
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	1,163	922
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	65,407	62,446
Available-for Sale, Gross Unrealized Losses, Total	$ 1,876	$ 1,126