Investment Securities - Schedule of Maturities of Investment Securities Available-for-Sale and Held-to-Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	$ 749,403	$ 739,404
Fair Value	756,731	740,124
Held-to-maturity Securities, Amortized Cost	138,060	126,168
Held-to-maturity Securities, Fair Value	138,138	126,171
Within One Year [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	3,995	10,000
Fair Value	4,000	10,014
Held-to-maturity Securities, Amortized Cost	1,712	3,608
Held-to-maturity Securities, Fair Value	1,714	3,608
After One Year But Within Five Years [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	30,032	20,020
Fair Value	30,112	19,946
Held-to-maturity Securities, Amortized Cost	3,925	6,217
Held-to-maturity Securities, Fair Value	3,913	6,217
After Five Years But Within Ten Years [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	192,473	134,453
Fair Value	194,009	133,395
Held-to-maturity Securities, Amortized Cost	7,964	9,733
Held-to-maturity Securities, Fair Value	8,053	9,736
After Ten Years [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	522,903	574,931
Fair Value	528,610	576,769
Held-to-maturity Securities, Amortized Cost	124,459	106,610
Held-to-maturity Securities, Fair Value	$ 124,458	$ 106,610