Impairment of Investment Securities - Pooled Trust Preferred Collateralized Debt Obligations (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($) Bank	Dec. 31, 2016 USD ($)
Impairment Of Investment Securities [Line Items]
Securities available for sale, at fair value	$ 778,644	$ 778,612
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	7,051	7,076
Unrealized Gain (Loss)	13,282	$ 18,546
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Impairment Of Investment Securities [Line Items]
Book Value	40,594
Securities available for sale, at fair value	34,629
Unrealized Gain (Loss)	5,965
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL IV [Member]
Impairment Of Investment Securities [Line Items]
Book Value	1,820
Securities available for sale, at fair value	1,393
Unrealized Gain (Loss)	$ 427
Debt Instrument, Credit Rating	Ba1/BB
Number of Banks (in banks) | Bank	6
Deferrals and Defaults as a % of Current Collateral	18.05%
Excess Subordination as a % of Current Performing Collateral	65.06%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL VIII [Member]
Impairment Of Investment Securities [Line Items]
Book Value	$ 2,013
Securities available for sale, at fair value	2,215
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	$ 202
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	$ 2,439
Securities available for sale, at fair value	2,023
Unrealized Gain (Loss)	$ 416
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	10.00%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL X [Member]
Impairment Of Investment Securities [Line Items]
Book Value	$ 1,831
Securities available for sale, at fair value	2,062
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	$ 231
Debt Instrument, Credit Rating	Caa1/C
Number of Banks (in banks) | Bank	42
Deferrals and Defaults as a % of Current Collateral	30.10%
Excess Subordination as a % of Current Performing Collateral	2.06%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL XII [Member]
Impairment Of Investment Securities [Line Items]
Book Value	$ 6,044
Securities available for sale, at fair value	5,064
Unrealized Gain (Loss)	$ 980
Debt Instrument, Credit Rating	B3/C
Number of Banks (in banks) | Bank	63
Deferrals and Defaults as a % of Current Collateral	23.26%
Excess Subordination as a % of Current Performing Collateral	0.00%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL XIII [Member]
Impairment Of Investment Securities [Line Items]
Book Value	$ 13,131
Securities available for sale, at fair value	11,292
Unrealized Gain (Loss)	$ 1,839
Debt Instrument, Credit Rating	Ba1/CCC
Number of Banks (in banks) | Bank	51
Deferrals and Defaults as a % of Current Collateral	8.88%
Excess Subordination as a % of Current Performing Collateral	48.56%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL XIV [Member]
Impairment Of Investment Securities [Line Items]
Book Value	$ 13,101
Securities available for sale, at fair value	10,282
Unrealized Gain (Loss)	$ 2,819
Debt Instrument, Credit Rating	Ba2/CCC
Number of Banks (in banks) | Bank	49
Deferrals and Defaults as a % of Current Collateral	12.95%
Excess Subordination as a % of Current Performing Collateral	40.28%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | MMCap I [Member]
Impairment Of Investment Securities [Line Items]
Book Value	$ 215
Securities available for sale, at fair value	298
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	$ 83
Debt Instrument, Credit Rating	Ca/C
Number of Banks (in banks) | Bank	8
Deferrals and Defaults as a % of Current Collateral	58.11%
Excess Subordination as a % of Current Performing Collateral	58.72%