Deposits - Composition of Deposits and Interest Expenses (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule Of Deposits And Borrowings Maturities [Line Items]
Balance	$ 3,908,842	$ 3,672,308	$ 3,638,428
Interest Expense	10,128	11,406	16,796
Noninterest-bearing Demand [Member]
Schedule Of Deposits And Borrowings Maturities [Line Items]
Balance	1,083,919	992,779	960,980
Interest-bearing Demand [Member]
Schedule Of Deposits And Borrowings Maturities [Line Items]
Balance	335,099	312,790	316,760
Interest Expense	19	75	146
Money Market [Member]
Schedule Of Deposits And Borrowings Maturities [Line Items]
Balance	376,612	281,403	361,233
Interest Expense	572	446	528
Savings [Member]
Schedule Of Deposits And Borrowings Maturities [Line Items]
Balance	1,027,095	994,805	965,571
Interest Expense	1,607	1,735	2,356
Certificates of Deposit [Member]
Schedule Of Deposits And Borrowings Maturities [Line Items]
Balance	1,086,117	1,090,531	1,033,884
Interest Expense	$ 7,930	$ 9,150	$ 13,766