Income Taxes - Deferred tax liabilities and deferred tax assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Benefit plans		$ 3,381
Accrued compensation and related costs	$ 5,868	5,558
Other accrued expenses	3,861	4,094
Unrealized losses on investment securities and partnerships	6,272
Share-based compensation	10,300	15,047
Unused state tax credits	2,618	2,788
State net operating loss carryforwards	7,266	7,235
Other	1,171	2,874
Total gross deferred assets	37,356	40,977
Deferred tax liabilities:
Property and equipment	(3,700)	(7,301)
Benefit plans	(1,872)
Identifiable intangible assets	(9,206)	(7,419)
Unrealized gain on investments securities and partnerships		(3,554)
Prepaid expenses	(2,478)	(1,679)
Other	(513)	(481)
Total gross deferred liabilities	(17,769)	(20,434)
Valuation allowance	(7,266)	(7,235)
Net deferred tax asset	12,321	13,308
Deferred tax assets, operating loss carryforwards, subject to expiration	7,300	7,200
Operating loss carryforward, valuation allowance	7,300	$ 7,200
State tax credit carryforwards that will expire between 2024 and 2034 if not utilized	2,300
State tax credit carryforwards that will expire in 2026 if not utilized	200
State tax credit carryforwards, indefinite carryforward	$ 100