Balance Sheet Offsetting - Financial Instruments Derivative Assets Liabilities and Resell Agreements Net of Amount Not Offset (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	$ 3,810	$ 3,798
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(3,308)	(3,565)
Gross Amounts Not Offset, Collateral, Derivatives, Financial Assets	(387)	(64)
Net Amount, Derivatives, Financial Assets	115	169
Net Amount Recognized, Resell Agreements, Financial Assets	9,642	9,642
Gross Amounts Not Offset, Financial Instruments, Resell Agreements, Financial Assets	0	0
Gross Amounts Not Offset, Collateral, Resell Agreements, Financial Assets	(9,642)	(9,642)
Net Amount, Resell Agreements, Financial Assets	0	0
Net Amount Recognized, Total Financial Assets	13,452	13,440
Gross Amounts Not Offset, Collateral, Financial Assets	(10,029)	(9,706)
Net Amount, Financial Assets	115	169
Net Amount Recognized, Derivatives, Financial Liabilities	27,375	32,534
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Liabilities	(3,308)	(3,565)
Gross Amounts Not offset, Collateral, Derivatives, Financial Liabilities	(22,768)	(27,347)
Net Amount, Derivatives, Financial Liabilities	1,299	1,622
Net Amount Recognized, Repurchase Agreements, Financial Liabilities	1,117,199	963,317
Gross Amounts Not Offset, Financial Instruments, Repurchase Agreements, Financial Liabilities	0	0
Gross Amounts Not Offset, Collateral, Repurchase Agreements, Financial Liabilities	(1,117,199)	(963,317)
Net Amount, Repurchase Agreements, Financial Liabilities	0	0
Net Amount Recognized, Financial Liabilities	1,144,574	995,851
Gross Amounts Not Offset, Collateral, Financial Liabilities	(1,139,967)	(990,664)
Net Amount, Financial Liabilities	1,299	1,622
Counterparty A [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	395	687
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(395)	(687)
Gross Amounts Not Offset, Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	7,397	11,233
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Liabilities	(395)	(687)
Gross Amounts Not offset, Collateral, Derivatives, Financial Liabilities	(7,002)	(10,026)
Net Amount, Derivatives, Financial Liabilities	0	520
Counterparty B [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	1,028	223
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(1,028)	(223)
Gross Amounts Not Offset, Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	4,466	6,867
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Liabilities	(1,028)	(223)
Gross Amounts Not offset, Collateral, Derivatives, Financial Liabilities	(3,101)	(6,344)
Net Amount, Derivatives, Financial Liabilities	337	300
Counterparty C [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	55	158
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(55)	(158)
Gross Amounts Not Offset, Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	1,520	4,578
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Liabilities	(55)	(158)
Gross Amounts Not offset, Collateral, Derivatives, Financial Liabilities	(1,450)	(4,415)
Net Amount, Derivatives, Financial Liabilities	15	5
Counterparty D [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	0	1,820
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	0	(1,820)
Gross Amounts Not Offset, Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	0	7,706
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Liabilities	0	(1,820)
Gross Amounts Not offset, Collateral, Derivatives, Financial Liabilities	0	(5,886)
Net Amount, Derivatives, Financial Liabilities	0	0
Other Counterparties [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	2,332	910
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(1,830)	(677)
Gross Amounts Not Offset, Collateral, Derivatives, Financial Assets	(387)	(64)
Net Amount, Derivatives, Financial Assets	115	169
Net Amount Recognized, Derivatives, Financial Liabilities	13,992	2,150
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Liabilities	(1,830)	(677)
Gross Amounts Not offset, Collateral, Derivatives, Financial Liabilities	(11,215)	(676)
Net Amount, Derivatives, Financial Liabilities	$ 947	$ 797