Comprehensive Income Changes in Accumulated Other Comprehensive Income By Component (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Net current-period other comprehensive income	[1]	$ 65	$ (700)	$ 699	$ 2,530
Unrealized Gains and Losses on Available for Sale Securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' Equity, Period Start		74	3,660	(382)	629
Other comprehensive income or loss before reclassifications		9	(810)	465	2,309
Amounts reclassified from accumulated other comprehensive income		(6)	0	(6)	(88)
Net current-period other comprehensive income		3	(810)	459	2,221
Stockholders' Equity, Period End		77	2,850	77	2,850
Amortization of Net Retirement Plan Actuarial Loss
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' Equity, Period Start		(5,556)	(7,690)	(5,737)	(7,893)
Other comprehensive income or loss before reclassifications		0	0	0	0
Amounts reclassified from accumulated other comprehensive income		64	111	245	314
Net current-period other comprehensive income		64	111	245	314
Stockholders' Equity, Period End		(5,492)	(7,579)	(5,492)	(7,579)
Accretion of Net Retirement Plan Prior Service Credit
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' Equity, Period Start		(718)	(712)	(715)	(708)
Other comprehensive income or loss before reclassifications		0	0	0	0
Amounts reclassified from accumulated other comprehensive income		(2)	(1)	(5)	(5)
Net current-period other comprehensive income		(2)	(1)	(5)	(5)
Stockholders' Equity, Period End		(720)	(713)	(720)	(713)
AOCI Including Portion Attributable to Noncontrolling Interest
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' Equity, Period Start		(6,200)	(4,742)	(6,834)	(7,972)
Other comprehensive income or loss before reclassifications		9	(810)	465	2,309
Amounts reclassified from accumulated other comprehensive income		56	110	234	221
Net current-period other comprehensive income		65	(700)	699	2,530
Stockholders' Equity, Period End		$ (6,135)	$ (5,442)	$ (6,135)	$ (5,442)

[1] These accumulated other comprehensive income components are included in the computation of net periodic pension cost.