Investment Securities - Summary of Amortized Cost and Estimated Fair Values of Available for Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Amortized Cost	$ 1,654,657	$ 1,277,639
Gross Unrealized Gains	11,446	10,912
Gross Unrealized Losses	16,469	29,337
Estimated Fair Value	1,649,634	1,259,214
Cumulative OTTI in AOCI	20,856	26,038
U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Investments [Line Items]
Amortized Cost	115,224	95,247
Gross Unrealized Gains	864	698
Gross Unrealized Losses	222	159
Estimated Fair Value	115,866	95,786
Cumulative OTTI in AOCI	0	0
State and Political Subdivisions [Member]
Schedule of Investments [Line Items]
Amortized Cost	305,096	196,350
Gross Unrealized Gains	2,567	1,364
Gross Unrealized Losses	2,522	4,902
Estimated Fair Value	305,141	192,812
Cumulative OTTI in AOCI	0	0
Residential Mortgage-Backed Securities, Agency [Member]
Schedule of Investments [Line Items]
Amortized Cost	715,003	585,208
Gross Unrealized Gains	3,819	3,999
Gross Unrealized Losses	4,122	5,111
Estimated Fair Value	714,700	584,096
Cumulative OTTI in AOCI	0	0
Residential Mortgage-Backed Securities, Non-agency [Member]
Schedule of Investments [Line Items]
Amortized Cost	5,259	6,629
Gross Unrealized Gains	587	426
Gross Unrealized Losses	0	12
Estimated Fair Value	5,846	7,043
Cumulative OTTI in AOCI	86	86
Commercial Mortgage-Backed Securities, Agency [Member]
Schedule of Investments [Line Items]
Amortized Cost	420,115	304,635
Gross Unrealized Gains	2,081	1,948
Gross Unrealized Losses	1,404	1,242
Estimated Fair Value	420,792	305,341
Cumulative OTTI in AOCI	0	0
Asset-Backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	13,422	217
Gross Unrealized Gains	10	0
Gross Unrealized Losses	3	0
Estimated Fair Value	13,429	217
Cumulative OTTI in AOCI	0	0
Trust Preferred Collateralized Debt Obligations [Member]
Schedule of Investments [Line Items]
Amortized Cost	38,186	48,558
Gross Unrealized Gains	317	729
Gross Unrealized Losses	6,844	15,735
Estimated Fair Value	31,659	33,552
Cumulative OTTI in AOCI	20,770	25,952
Single Issue Trust Preferred Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	13,404	13,363
Gross Unrealized Gains	404	284
Gross Unrealized Losses	1,341	2,170
Estimated Fair Value	12,467	11,477
Cumulative OTTI in AOCI	0	0
Corporate Bonds [Member]
Schedule of Investments [Line Items]
Amortized Cost	18,998	14,996
Gross Unrealized Gains	256	66
Gross Unrealized Losses	0	0
Estimated Fair Value	19,254	15,062
Cumulative OTTI in AOCI	0	0
Marketable Equity Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	9,950	12,436
Gross Unrealized Gains	541	1,398
Gross Unrealized Losses	11	6
Estimated Fair Value	10,480	13,828
Cumulative OTTI in AOCI	$ 0	$ 0

Total of accumulated other comprehensive loss related to that portion of other than temporary impairment (OTTI) due to factors other than credit losses on debt securities categorized either as available-for-sale or held-to-maturity 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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-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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