Accumulated Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive Income (Loss) by Component, Net of Tax (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ 59,091,000	$ 54,073,000
Other comprehensive income before reclassification	(1,038,000)	258,000
Amounts reclassified from accumulated other comprehensive income (loss)	(1,362,000)	(956,000)
Net of tax	(1,015,000)	258,000
Balance at end of period	58,877,000	54,761,000
Amount Reclassified From Accumulated Other Comprehensive Income
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from accumulated other comprehensive income (loss)	23,000	0
Unrealized Gains and Losses on Available-for-Sale Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(866,000)	(679,000)
Cumulative effect of change in accounting principle for marketable equity securities, net of tax			$ (187,000)
Other comprehensive income before reclassification	(1,038,000)	258,000
Net of tax	(1,015,000)	258,000
Balance at end of period	(1,881,000)	(421,000)
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)	23,000	0
Defined Benefit Pension Items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(4,839,000)	(3,812,000)
Cumulative effect of change in accounting principle for marketable equity securities, net of tax			0
Other comprehensive income before reclassification	0	0
Net of tax	0	0
Balance at end of period	(4,839,000)	(3,812,000)
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
Totals
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(5,705,000)	(4,491,000)
Cumulative effect of change in accounting principle for marketable equity securities, net of tax			$ (187,000)
Balance at end of period	(6,720,000)	$ (4,233,000)
Previously Reported
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	59,091,000
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	(679,000)
Previously Reported | Defined Benefit Pension Items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(4,839,000)
Previously Reported | Totals
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ (5,518,000)