Deposits - Composition of Deposits and Interest Expenses (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Deposits And Borrowings Maturities [Line Items]
Balance	$ 5,427,891	$ 5,272,377	$ 4,876,611
Interest Expense	25,330	19,692	12,944
Noninterest-bearing demand
Schedule Of Deposits And Borrowings Maturities [Line Items]
Balance	1,387,712	1,263,833	1,227,766
Interest-bearing demand
Schedule Of Deposits And Borrowings Maturities [Line Items]
Balance	603,141	638,300	616,188
Interest Expense	67	111	818
Money market
Schedule Of Deposits And Borrowings Maturities [Line Items]
Balance	1,146,156	936,461	605,184
Interest Expense	9,204	4,199	1,299
Savings
Schedule Of Deposits And Borrowings Maturities [Line Items]
Balance	893,119	1,050,131	1,061,265
Interest Expense	2,081	2,002	1,712
Certificates of deposit
Schedule Of Deposits And Borrowings Maturities [Line Items]
Balance	1,397,763	1,383,652	1,366,208
Interest Expense	$ 13,978	$ 13,380	$ 9,115