Investment Securities - Summary of Amortized Cost and Estimated Fair Values of Available for Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Amortized Cost	$ 2,730,350	$ 2,426,924
Gross Unrealized Gains	90,769	28,912
Gross Unrealized Losses	21,178	18,540
Allowance For Credit Losses	0
Estimated Fair Value	2,799,941	2,437,296
Cumulative OTTI in AOCI		928
U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Investments [Line Items]
Amortized Cost	66,394	58,127
Gross Unrealized Gains	912	555
Gross Unrealized Losses	3	6
Allowance For Credit Losses	0
Estimated Fair Value	67,303	58,676
Cumulative OTTI in AOCI		0
State and Political Subdivisions [Member]
Schedule of Investments [Line Items]
Amortized Cost	502,293	272,014
Gross Unrealized Gains	18,823	3,644
Gross Unrealized Losses	236	3,296
Allowance For Credit Losses	0
Estimated Fair Value	520,880	272,362
Cumulative OTTI in AOCI		0
Residential Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	819,073	826,857
Gross Unrealized Gains	29,209	10,923
Gross Unrealized Losses	51	1,246
Allowance For Credit Losses	0
Estimated Fair Value	848,231	836,534
Cumulative OTTI in AOCI		0
Residential Mortgage-Backed Securities, Non-agency [Member]
Schedule of Investments [Line Items]
Amortized Cost	61,954	3,429
Gross Unrealized Gains	1,011	404
Gross Unrealized Losses	0	0
Allowance For Credit Losses	0
Estimated Fair Value	62,965	3,833
Cumulative OTTI in AOCI		86
Commercial Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	591,303	609,461
Gross Unrealized Gains	31,581	8,319
Gross Unrealized Losses	99	2,807
Allowance For Credit Losses	0
Estimated Fair Value	622,785	614,973
Cumulative OTTI in AOCI		0
Asset-Backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	293,293	284,390
Gross Unrealized Gains	0	0
Gross Unrealized Losses	15,572	8,251
Allowance For Credit Losses	0
Estimated Fair Value	277,721	276,139
Cumulative OTTI in AOCI		0
Trust Preferred Collateralized Debt Obligations [Member]
Schedule of Investments [Line Items]
Amortized Cost	11,895	6,045
Gross Unrealized Gains	2,644	0
Gross Unrealized Losses	1,702	1,342
Allowance For Credit Losses	0
Estimated Fair Value	12,837	4,703
Cumulative OTTI in AOCI		842
Single Issue Trust Preferred Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	18,211	18,196
Gross Unrealized Gains	159	170
Gross Unrealized Losses	2,179	1,592
Allowance For Credit Losses	0
Estimated Fair Value	16,191	16,774
Cumulative OTTI in AOCI		0
Corporate Bonds [Member]
Schedule of Investments [Line Items]
Amortized Cost	365,934	348,405
Gross Unrealized Gains	6,430	4,897
Gross Unrealized Losses	1,336	0
Allowance For Credit Losses	0
Estimated Fair Value	$ 371,028	353,302
Cumulative OTTI in AOCI		$ 0

Accumulated other comprehensive loss related to portion of other than temporary impairment (OTTI) due to factors other than credit losses on debt securities categorized as available-for-sale that an entity does not intend to sell and which it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis of the debt security. The amount represented by this item is net of tax, as applicable.

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