Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	$ 337,721	$ 218,074
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	3,813	972
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	26,320	28,931
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	574	403
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	364,041	247,005
Held-to-maturity securities, Gross Unrealized Losses, Total	4,387	1,375
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	228,835	135,576
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	3,314	520
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	65,372	76,085
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	2,710	2,194
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	294,207	211,661
Available-for Sale, Gross Unrealized Losses, Total	6,024	2,714
Fair Value of Investments with Unrealized Losses, Less Than 12 months	566,556	353,650
Gross Unrealized Loss, Less Than 12 Months	7,127	1,492
Fair Value of Investments with Unrealized Losses, 12 Months or Greater	91,692	105,016
Gross Unrealized Losses, 12 Months or Greater	3,284	2,597
Fair Value of Investments with Unrealized Losses, Total	658,248	458,666
Gross Unrealized Losses, Total	10,411	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	204,459	110,698
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	2,568	654
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	12,424	19,895
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	323	301
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	216,883	130,593
Held-to-maturity securities, Gross Unrealized Losses, Total	2,891	955
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	77,359	23,365
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	1,356	86
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	15,476	19,153
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	1,139	491
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	92,835	42,518
Available-for Sale, Gross Unrealized Losses, Total	2,495	577
Corporate Bonds [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	86,816	56,425
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	703	156
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	2,237	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	89,053	60,546
Held-to-maturity securities, Gross Unrealized Losses, Total	724	167
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	121,848	82,795
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	1,016	171
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	1,021	5,888
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	35	41
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	122,869	88,683
Available-for Sale, Gross Unrealized Losses, Total	1,051	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	2,524	2,798
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	71	40
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	2,524	2,798
Held-to-maturity securities, Gross Unrealized Losses, Total	71	40
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	11,557	14,686
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	312	59
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	1,805	3,328
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	56	740
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	13,362	18,014
Available-for Sale, Gross Unrealized Losses, Total	368	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	43,357	48,153
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	461	122
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	11,659	3,948
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	230	76
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	55,016	52,101
Held-to-maturity securities, Gross Unrealized Losses, Total	691	198
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	18,071	14,730
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	630	204
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	47,070	47,716
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	1,480	922
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	65,141	62,446
Available-for Sale, Gross Unrealized Losses, Total	2,110	1,126
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	565
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	10
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater		967
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater		15
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	565	967
Held-to-maturity securities, Gross Unrealized Losses, Total	$ 10	$ 15