- Composition of Short-Term Borrowings, Interest Expense and Weighted Average Interest Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Short-term Debt [Line Items]
Securities available-for-sale, amortized cost	$ 686,205	$ 650,782
Securities available-for-sale, at fair value	693,487	660,963
Balance	$ 710,832	$ 418,086	$ 320,605
Weighted Average Interest Rate	0.70%	0.44%	0.27%
Interest Expense	$ 2,718	$ 936	$ 514
REPOs
Short-term Debt [Line Items]
Balance	$ 50,832	$ 62,086	$ 30,605
Weighted Average Interest Rate	0.01%	0.01%	0.01%
Interest Expense	$ 5	$ 4	$ 3
FHLB advances
Short-term Debt [Line Items]
Balance	$ 660,000	$ 356,000	$ 290,000
Weighted Average Interest Rate	0.76%	0.52%	0.31%
Interest Expense	$ 2,713	$ 932	$ 511
Securities Pledged as Collateral
Short-term Debt [Line Items]
Securities available-for-sale, amortized cost	53,100	67,000
Securities available-for-sale, at fair value	$ 52,900	$ 66,900