Derivatives and Hedging Activities - Offsetting Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Derivative Asset, Gross Amounts Recognized	$ 10,721	$ 2,749
Derivative Asset, Gross Amounts Offset in the Balance Sheet	0	0
Derivative Asset, Net Amounts in the Balance Sheet	10,721	2,749
Derivative Asset, Financial Instruments	1,530	479
Derivative Asset, Fair Value of Financial Collateral in the Balance Sheet	0	0
Derivative Asset, Net Amount	9,191	2,270
Total assets, Gross Amounts Recognized	10,721	2,749
Total assets, Gross Amounts Offset in the Balance Sheet	0	0
Total assets, Net Amounts in the Balance Sheet	10,721	2,749
Total assets, Financial Instruments	1,530	479
Total assets, Fair Value of Financial Collateral in the Balance Sheet	0	0
Total assets, Net Amount	9,191	2,270
Derivative liability, Gross Amounts Recognized	9,099	1,314
Derivative liability, Gross Amounts Offset in the Balance Sheet	0	0
Derivative liability, Net Amounts in the Balance Sheet	9,099	1,314
Derivative liability, Financial Instruments	1,530	479
Derivative liability, Fair Value of Financial Collateral in the Balance Sheet	7,569	0
Derivative liability, Net Amount	0	835
Repurchase agreements, Gross Amounts Recognized	635,288	537,556
Repurchase agreements, Gross Amounts Offset in the Balance Sheet	0	0
Repurchase agreements, Net Amounts in the Balance Sheet	635,288	537,556
Repurchase agreements, Financial Instruments	0	0
Repurchase agreements, Fair Value of Financial Collateral in the Balance Sheet	635,288	537,556
Repurchase agreements, Net Amount	0	0
Total liabilities, Gross Amounts Recognized	644,387	538,870
Total liabilities, Gross Amounts Offset in the Balance Sheet	0	0
Total liabilities, Net Amounts in the Balance Sheet	644,387	538,870
Total liabilities, Financial Instruments	1,530	479
Total liabilities, Fair Value of Financial Collateral in the Balance Sheet	642,857	537,556
Total liabilities, Net Amount	0	835
Interest Rate Swap
Derivative [Line Items]
Derivative Asset, Gross Amounts Recognized	8,764	1,332
Derivative Asset, Gross Amounts Offset in the Balance Sheet	0	0
Derivative Asset, Net Amounts in the Balance Sheet	8,764	1,332
Derivative Asset, Financial Instruments	1,530	479
Derivative Asset, Fair Value of Financial Collateral in the Balance Sheet	0	0
Derivative Asset, Net Amount	7,234	853
Mortgage Related Derivatives
Derivative [Line Items]
Derivative Asset, Gross Amounts Recognized	1,957	1,417
Derivative Asset, Gross Amounts Offset in the Balance Sheet	0	0
Derivative Asset, Net Amounts in the Balance Sheet	1,957	1,417
Derivative Asset, Financial Instruments	0	0
Derivative Asset, Fair Value of Financial Collateral in the Balance Sheet	0	0
Derivative Asset, Net Amount	$ 1,957	$ 1,417