Other Comprehensive Income (Loss) - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances at beginning	$ 663,054	$ 620,871	$ 576,202
Balances at ending	717,689	663,054	620,871
Available-for-Sale Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances at beginning	4,039	(23,589)	(13,005)
Other comprehensive income (loss)	16,570	27,628	(10,649)
Balances at ending	20,609	4,039	(23,589)
Employee Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances at beginning	(47,603)	(39,576)	(38,291)
Other comprehensive income (loss)	(5,080)	(8,027)	(1,285)
Balances at ending	(52,683)	(47,603)	(39,576)
Accumulated Other Comprehensive Income (loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances at beginning	(43,564)	(63,165)	(51,296)
Other comprehensive income (loss)	11,490	19,601	(11,934)
Balances at ending	$ (32,074)	$ (43,564)	(63,165)
Adoption of ASU 2016-01 | Available-for-Sale Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification due to adoption of ASU			65
Adoption of ASU 2016-01 | Employee Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification due to adoption of ASU			0
Adoption of ASU 2016-01 | Accumulated Other Comprehensive Income (loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification due to adoption of ASU			$ 65