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Investment Securities - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Maturity of contract		less than one year
Approximate year of maturity		30 years
Gross Unrealized Gains (Losses)		$ 14,342	$ 18,546
Securities available for sale		731,358	778,612
Amortized Cost		740,846	790,082
Available-for-sale securities pledged as collateral		569,000	445,800
Held-to-maturity securities pledged as collateral		338,300	119,200
Sales [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Gross Realized Losses		359	305	$ 0
Available-for-sale Securities, Gross Realized Losses		316	277	284
Sales [Member] | US Government Agencies Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost				75,000
Calls And Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Gains		5,057	589	131
Available-for-sale Securities, Gross Realized Losses		60	0	$ 0
Calls And Maturities [Member] | Pre TSL VII [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Gross Realized Losses	$ 712		589
Gross Gains		700
Amortized Cost			3,119
Calls And Maturities [Member] | Pre TSL XIII [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Gross Realized Losses		$ 4,329
Amortized Cost			$ 13,171

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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