Securities - Schedule of Unrealized Losses on Debt Securities (Detail) $ in Thousands	Jun. 30, 2018 USD ($) Security	Dec. 31, 2017 USD ($) Security
US Treasury Securities [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 9,913
Less than 12 months, Unrealized Losses	$ (13)
Less than 12 months, Number of Securities | Security	1
Fair Value, Total	$ 9,913
Unrealized Losses, Total	$ (13)
Number of Securities Total | Security	1
U.S. Government Sponsored Entities and Agencies [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 71,179	$ 24,776
Less than 12 months, Unrealized Losses	$ (1,885)	$ (160)
Less than 12 months, Number of Securities | Security	23	4
12 months or more, Fair Value	$ 40,491	$ 42,248
12 months or more, Unrealized Losses	$ (975)	$ (771)
12 months or more, Number of Securities | Security	8	8
Fair Value, Total	$ 111,670	$ 67,024
Unrealized Losses, Total	$ (2,860)	$ (931)
Number of Securities Total | Security	31	12
Residential Mortgage-Backed Securities and Collateralized Mortgage Obligations of Government Sponsored Entities and Agencies [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 864,036	$ 423,794
Less than 12 months, Unrealized Losses	$ (16,625)	$ (5,039)
Less than 12 months, Number of Securities | Security	160	87
12 months or more, Fair Value	$ 610,641	$ 637,461
12 months or more, Unrealized Losses	$ (29,733)	$ (16,977)
12 months or more, Number of Securities | Security	201	193
Fair Value, Total	$ 1,474,677	$ 1,061,255
Unrealized Losses, Total	$ (46,358)	$ (22,016)
Number of Securities Total | Security	361	280
Commercial Mortgage-Backed Securities and Collateralized Mortgage Obligations of Government Sponsored Entities and Agencies [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 104,546	$ 79,061
Less than 12 months, Unrealized Losses	$ (3,531)	$ (1,089)
Less than 12 months, Number of Securities | Security	13	10
12 months or more, Fair Value	$ 23,225	$ 27,852
12 months or more, Unrealized Losses	$ (981)	$ (496)
12 months or more, Number of Securities | Security	4	6
Fair Value, Total	$ 127,771	$ 106,913
Unrealized Losses, Total	$ (4,512)	$ (1,585)
Number of Securities Total | Security	17	16
Obligations of State and Political Subdivisions [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 357,070	$ 132,831
Less than 12 months, Unrealized Losses	$ (5,019)	$ (852)
Less than 12 months, Number of Securities | Security	571	210
12 months or more, Fair Value	$ 76,447	$ 77,554
12 months or more, Unrealized Losses	$ (2,256)	$ (1,217)
12 months or more, Number of Securities | Security	163	160
Fair Value, Total	$ 433,517	$ 210,385
Unrealized Losses, Total	$ (7,275)	$ (2,069)
Number of Securities Total | Security	734	370
Corporate Debt Securities [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 43,559	$ 4,015
Less than 12 months, Unrealized Losses	$ (791)	$ (19)
Less than 12 months, Number of Securities | Security	21	1
12 months or more, Fair Value	$ 1,948	$ 1,948
12 months or more, Unrealized Losses	$ (41)	$ (40)
12 months or more, Number of Securities | Security	1	1
Fair Value, Total	$ 45,507	$ 5,963
Unrealized Losses, Total	$ (832)	$ (59)
Number of Securities Total | Security	22	2
Total Temporarily Impaired Securities [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 1,450,303	$ 664,477
Less than 12 months, Unrealized Losses	$ (27,864)	$ (7,159)
Less than 12 months, Number of Securities | Security	789	312
12 months or more, Fair Value	$ 752,752	$ 787,063
12 months or more, Unrealized Losses	$ (33,986)	$ (19,501)
12 months or more, Number of Securities | Security	377	368
Fair Value, Total	$ 2,203,055	$ 1,451,540
Unrealized Losses, Total	$ (61,850)	$ (26,660)
Number of Securities Total | Security	1,166	680