Comprehensive Income Changes in Accumulated Other Comprehensive Income By Component (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Net current-period other comprehensive income (loss)		$ (2,440)	$ 154
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	[1]	(1,250)	(382)
Other comprehensive income or loss before reclassifications	[1]	(2,485)	47
Amounts reclassified from accumulated other comprehensive income	[1]	0	0
Net current-period other comprehensive income (loss)	[1]	(2,485)	47
Reclassification due to the adoption of ASU 2016-01	[1]			$ (331)
Stockholders' Equity, Period End	[1]	(4,066)	(335)
Amortization of Net Retirement Plan Actuarial Gain (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' Equity, Period Start	[1]	(6,380)	(5,737)
Other comprehensive income or loss before reclassifications	[1]	0	0
Amounts reclassified from accumulated other comprehensive income	[1]	46	109
Net current-period other comprehensive income (loss)	[1]	46	109
Stockholders' Equity, Period End	[1]	(6,334)	(5,628)
Accretion of Net Retirement Plan Prior Service (Cost) Credit
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' Equity, Period Start	[1]	(884)	(715)
Other comprehensive income or loss before reclassifications	[1]	0	0
Amounts reclassified from accumulated other comprehensive income	[1]	(1)	(2)
Net current-period other comprehensive income (loss)	[1]	(1)	(2)
Stockholders' Equity, Period End	[1]	(885)	(717)
Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' Equity, Period Start	[1]	(8,514)	(6,834)
Other comprehensive income or loss before reclassifications	[1]	(2,485)	47
Amounts reclassified from accumulated other comprehensive income	[1]	45	107
Net current-period other comprehensive income (loss)	[1]	(2,440)	154
Reclassification due to the adoption of ASU 2016-01	[1]			$ (331)
Stockholders' Equity, Period End	[1]	$ (11,285)	$ (6,680)

[1] All amounts are net of tax. Amounts in parentheses indicate debits.