Impairment of Investment Securities - Pooled Trust Preferred Collateralized Debt Obligations (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) Bank	Dec. 31, 2017 USD ($)
Impairment Of Investment Securities [Line Items]
Securities available for sale, at fair value	$ 812,877	$ 731,358
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	4,474	4,854
Unrealized Gain (Loss)	20,785	$ 14,342
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Impairment Of Investment Securities [Line Items]
Book Value	13,602
Securities available for sale, at fair value	14,132
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	530
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL IV [Member]
Impairment Of Investment Securities [Line Items]
Book Value	933
Securities available for sale, at fair value	738
Unrealized Gain (Loss)	$ 195
Debt Instrument, Credit Rating	Ba1/BB
Number of Banks (in banks) | Bank	5
Deferrals and Defaults as a % of Current Collateral	0.00%
Excess Subordination as a % of Current Performing Collateral	113.79%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL VIII [Member]
Impairment Of Investment Securities [Line Items]
Book Value	$ 2,072
Securities available for sale, at fair value	2,295
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	$ 223
Debt Instrument, Credit Rating	C/C
Number of Banks (in banks) | Bank	26
Deferrals and Defaults as a % of Current Collateral	38.52%
Excess Subordination as a % of Current Performing Collateral	0.00%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL IX [Member]
Impairment Of Investment Securities [Line Items]
Book Value	$ 2,457
Securities available for sale, at fair value	3,000
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	$ 543
Debt Instrument, Credit Rating	B1/C
Number of Banks (in banks) | Bank	37
Deferrals and Defaults as a % of Current Collateral	27.83%
Excess Subordination as a % of Current Performing Collateral	19.46%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL X [Member]
Impairment Of Investment Securities [Line Items]
Book Value	$ 1,894
Securities available for sale, at fair value	2,301
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	$ 407
Debt Instrument, Credit Rating	Caa1/C
Number of Banks (in banks) | Bank	41
Deferrals and Defaults as a % of Current Collateral	26.29%
Excess Subordination as a % of Current Performing Collateral	1.73%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL XII [Member]
Impairment Of Investment Securities [Line Items]
Book Value	$ 6,151
Securities available for sale, at fair value	5,624
Unrealized Gain (Loss)	$ 527
Debt Instrument, Credit Rating	B3/C
Number of Banks (in banks) | Bank	64
Deferrals and Defaults as a % of Current Collateral	23.39%
Excess Subordination as a % of Current Performing Collateral	0.00%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | MMCap I [Member]
Impairment Of Investment Securities [Line Items]
Book Value	$ 95
Securities available for sale, at fair value	174
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	$ 79
Debt Instrument, Credit Rating	Ca/C
Number of Banks (in banks) | Bank	7
Deferrals and Defaults as a % of Current Collateral	69.35%
Excess Subordination as a % of Current Performing Collateral	69.99%