Accumulated Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive Income (Loss) by Component, Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			$ 59,091	$ 54,073
Other comprehensive income before reclassification	$ (237)	$ 10	(1,275)	268
Amounts reclassified from accumulated other comprehensive income (loss)	(1,413)	(1,041)	(2,775)	(1,997)
Net of tax	(236)	114	(1,251)	372
Balance at end of period	59,493	56,647	59,493	56,647
Amount Reclassified From Accumulated Other Comprehensive Income
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from accumulated other comprehensive income (loss)	1	104	24	104
Unrealized Gains and Losses on Available-for-Sale Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(1,881)	(421)	(866)	(679)
Cumulative effect of change in accounting principle for marketable equity securities, net of tax					$ (187)
Other comprehensive income before reclassification	(237)	10	(1,275)	268
Net of tax	(236)	114	(1,251)	372
Balance at end of period	(2,117)	(307)	(2,117)	(307)
Unrealized Gains and Losses on Available-for-Sale Securities | Amount Reclassified From Accumulated Other Comprehensive Income
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from accumulated other comprehensive income (loss)	1	104	24	104
Defined Benefit Pension Items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(4,839)	(3,812)	(4,839)	(3,812)
Cumulative effect of change in accounting principle for marketable equity securities, net of tax					0
Other comprehensive income before reclassification	0	0	0	0
Net of tax	0	0	0	0
Balance at end of period	(4,839)	(3,812)	(4,839)	(3,812)
Defined Benefit Pension Items | Amount Reclassified From Accumulated Other Comprehensive Income
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from accumulated other comprehensive income (loss)	0	0	0	0
Totals
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(6,720)	(4,233)	(5,705)	(4,491)
Cumulative effect of change in accounting principle for marketable equity securities, net of tax					$ (187)
Balance at end of period	$ (6,956)	$ (4,119)	(6,956)	$ (4,119)
Previously Reported
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			59,091
Previously Reported | Unrealized Gains and Losses on Available-for-Sale Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			(679)
Previously Reported | Defined Benefit Pension Items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			(4,839)
Previously Reported | Totals
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			$ (5,518)