Accumulated Other Comprehensive Income (Loss) (''AOCI'') - Amount Reclassified from AOCI (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amount Reclassified from AOCI
Gain (loss) on sales of securities									$ (6)	$ 1,099	$ 642
Interest income on taxable securities									13,724	10,432	6,707
Income tax expense	$ (12,719)	$ (5,249)	$ (4,569)	$ (3,934)	$ (4,431)	$ (4,054)	$ (4,377)	$ (3,726)	(26,471)	(16,588)	(10,104)
Net income	$ 1,258	$ 8,583	$ 7,449	$ 6,538	$ 7,215	$ 6,765	$ 7,295	$ 6,106	23,828	27,381	16,497
Unrealized Gains on Available-for-Sale Securities and I/O Strips
Amount Reclassified from AOCI
Net of tax									(3)	638	372
Unrealized Gains on Available-for-Sale Securities and I/O Strips | Amount Reclassified from AOCI
Amount Reclassified from AOCI
Gain (loss) on sales of securities									(6)	1,099	642
Income tax expense									3	(461)	(270)
Net income									(3)	638	372
Unamortized Unrealized Gain on Available-for-Sale Securities Reclassified to Held-to-Maturity
Amount Reclassified from AOCI
Net of tax									29	67	32
Unamortized Unrealized Gain on Available-for-Sale Securities Reclassified to Held-to-Maturity | Amount Reclassified from AOCI
Amount Reclassified from AOCI
Interest income on taxable securities									51	116	55
Income tax expense									(22)	(49)	(23)
Net income									29	67	32
Defined Benefit Pension Plan Items
Amount Reclassified from AOCI
Net of tax									(119)	(109)	(158)
Defined Benefit Pension Plan Items | Amount Reclassified from AOCI
Amount Reclassified from AOCI
Income before income tax									(205)	(188)	(273)
Income tax benefit									86	79	115
Net of tax									(119)	(109)	(158)
Prior transition obligation | Amount Reclassified from AOCI
Amount Reclassified from AOCI
Income before income tax									71	51	113
Actuarial losses | Amount Reclassified from AOCI
Amount Reclassified from AOCI
Income before income tax									(276)	(239)	(386)
Accumulated Other Comprehensive Income / (Loss)
Amount Reclassified from AOCI
Net of tax									(93)	596	246
Accumulated Other Comprehensive Income / (Loss) | Amount Reclassified from AOCI
Amount Reclassified from AOCI
Net of tax									$ (93)	$ 596	$ 246