Derivatives and Hedging Activities - Schedule of Master Netting Arrangements (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financial Assets
Gross Amounts Recognized	$ 8,800	$ 2,700
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	8,800	2,700
Financial Instruments	2,400	500
Fair Value of Financial Collateral in the Balance Sheet	0	0
Net Amount	6,400	2,200
Offsetting Derivative Liabilities [Abstract]
Gross Amounts Recognized	7,900	1,300
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	7,900	1,300
Financial Instruments	2,400	500
Fair Value of Financial Collateral in the Balance Sheet	3,300	0
Net Amount	2,200	800
Repurchase agreements
Gross Amounts Recognized	643,000	537,600
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	642,961	537,556
Financial Instruments	0	0
Fair Value of Financial Collateral in the Balance Sheet	643,000	537,600
Net Amount	0	0
Offsetting Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned [Abstract]
Gross Amounts Recognized	650,900	538,900
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	650,900	538,900
Financial Instruments	2,400	500
Fair Value of Financial Collateral in the Balance Sheet	646,300	537,600
Net Amount	2,200	800
Interest rate swap contracts
Financial Assets
Gross Amounts Recognized	7,500	1,300
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	7,500	1,300
Financial Instruments	2,400	500
Fair Value of Financial Collateral in the Balance Sheet	0	0
Net Amount	5,100	800
Offsetting Derivative Liabilities [Abstract]
Gross Amounts Recognized	7,800	1,300
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	7,800	1,300
Financial Instruments	2,400	500
Fair Value of Financial Collateral in the Balance Sheet	3,300	0
Net Amount	2,100	800
Mortgage related derivatives
Financial Assets
Gross Amounts Recognized	1,300	1,400
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	1,300	1,400
Financial Instruments	0	0
Fair Value of Financial Collateral in the Balance Sheet	0	0
Net Amount	1,300	$ 1,400
Offsetting Derivative Liabilities [Abstract]
Gross Amounts Recognized	100
Gross Amounts Offset in the Balance Sheet	0
Net Amounts in the Balance Sheet	100
Financial Instruments	0
Fair Value of Financial Collateral in the Balance Sheet	0
Net Amount	$ 100