Other Comprehensive Income (Loss) - Activity in AOCI (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances at Beginning	$ 663,054	$ 620,871
Other comprehensive income	22,293	12,215
Balances at Ending	682,597	647,267
Available-for-Sale Debt Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances at Beginning	4,039	(23,589)
Other comprehensive income (loss) before reclassifications	22,123	11,894
Amounts reclassified from accumulated other comprehensive (loss) income	(324)	0
Other comprehensive income	21,799	11,894
Balances at Ending	25,838	(11,695)
Employee Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances at Beginning	(47,603)	(39,576)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive (loss) income	494	321
Other comprehensive income	494	321
Balances at Ending	(47,109)	(39,255)
Accumulated Other Comprehensive (Loss) Income
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances at Beginning	(43,564)	(63,165)
Other comprehensive income (loss) before reclassifications	22,123	11,894
Amounts reclassified from accumulated other comprehensive (loss) income	170	321
Other comprehensive income	22,293	12,215
Balances at Ending	$ (21,271)	$ (50,950)