Securities - Schedule of Unrealized Losses on Investment Securities (Detail) $ in Thousands	Dec. 31, 2016 USD ($) Security	Dec. 31, 2015 USD ($) Security
U.S. Government Sponsored Entities and Agencies [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 58,108	$ 49,826
Less than 12 months, Unrealized Losses	$ (1,177)	$ (403)
Less than 12 months, Number of Securities | Security	11	11
Fair Value, Total	$ 58,108	$ 49,826
Unrealized Losses, Total	$ (1,177)	$ (403)
Number of Securities Total | Security	11	11
Residential Mortgage-Backed Securities and Collateralized Mortgage Obligations of Government Agencies [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 1,057,343	$ 1,003,397
Less than 12 months, Unrealized Losses	$ (18,558)	$ (10,981)
Less than 12 months, Number of Securities | Security	246	187
12 months or more, Fair Value	$ 59,518	$ 146,182
12 months or more, Unrealized Losses	$ (2,214)	$ (4,929)
12 months or more, Number of Securities | Security	16	31
Fair Value, Total	$ 1,116,861	$ 1,149,579
Unrealized Losses, Total	$ (20,772)	$ (15,910)
Number of Securities Total | Security	262	218
Obligations of State and Political Subdivisions [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 364,583	$ 58,705
Less than 12 months, Unrealized Losses	$ (7,121)	$ (400)
Less than 12 months, Number of Securities | Security	604	76
12 months or more, Fair Value	$ 2,047	$ 23,691
12 months or more, Unrealized Losses	$ (67)	$ (372)
12 months or more, Number of Securities | Security	3	29
Fair Value, Total	$ 366,630	$ 82,396
Unrealized Losses, Total	$ (7,188)	$ (772)
Number of Securities Total | Security	607	105
Corporate Debt Securities [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 10,011	$ 41,326
Less than 12 months, Unrealized Losses	$ (78)	$ (541)
Less than 12 months, Number of Securities | Security	3	12
12 months or more, Fair Value	$ 5,973	$ 1,931
12 months or more, Unrealized Losses	$ (50)	$ (55)
12 months or more, Number of Securities | Security	2	1
Fair Value, Total	$ 15,984	$ 43,257
Unrealized Losses, Total	$ (128)	$ (596)
Number of Securities Total | Security	5	13
Equity Securities [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 2,938	$ 1,378
Less than 12 months, Unrealized Losses	$ (24)	$ (2)
Less than 12 months, Number of Securities | Security	2	1
Fair Value, Total	$ 2,938	$ 1,378
Unrealized Losses, Total	$ (24)	$ (2)
Number of Securities Total | Security	2	1
Total Temporarily Impaired Securities [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 1,492,983	$ 1,154,632
Less than 12 months, Unrealized Losses	$ (26,958)	$ (12,327)
Less than 12 months, Number of Securities | Security	866	287
12 months or more, Fair Value	$ 67,538	$ 171,804
12 months or more, Unrealized Losses	$ (2,331)	$ (5,356)
12 months or more, Number of Securities | Security	21	61
Fair Value, Total	$ 1,560,521	$ 1,326,436
Unrealized Losses, Total	$ (29,289)	$ (17,683)
Number of Securities Total | Security	887	348