Investment Securities - Schedule of Maturities of Investment Securities Available-for-Sale and Held-to-Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	$ 784,307	$ 739,404
Fair Value	781,746	740,124
Held-to-maturity Securities, Amortized Cost	120,697	126,168
Held-to-maturity Securities, Fair Value	118,804	126,171
Within One Year [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	3,000	10,000
Fair Value	3,001	10,014
Held-to-maturity Securities, Amortized Cost	2,692	3,608
Held-to-maturity Securities, Fair Value	2,694	3,608
After One Year But Within Five Years [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	29,032	20,020
Fair Value	29,024	19,946
Held-to-maturity Securities, Amortized Cost	3,016	6,217
Held-to-maturity Securities, Fair Value	2,967	6,217
After Five Years But Within Ten Years [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	181,317	134,453
Fair Value	179,384	133,395
Held-to-maturity Securities, Amortized Cost	9,278	9,733
Held-to-maturity Securities, Fair Value	9,319	9,736
After Ten Years [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	570,958	574,931
Fair Value	570,337	576,769
Held-to-maturity Securities, Amortized Cost	105,711	106,610
Held-to-maturity Securities, Fair Value	$ 103,824	$ 106,610