Income Taxes - Deferred income taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Accounts receivable allowance	$ 1,163	$ 1,534
Accrued compensation	9,553	10,184
Accrued pension liabilities	9,116	13,496
Self-insured risks	4,301	4,933
Deferred revenues	4,249	5,868
Accrued rent	0	3,115
Interest	5,399	5,665
Tax credit carryforwards	11,634	14,404
Loss carryforwards	25,523	25,745
Lease liability	29,785
Other	3,436	1,152
Gross deferred income tax assets	104,159	86,096
Unbilled revenues	6,522	7,725
Depreciation and amortization	28,025	32,510
Lease right-of-use asset	25,865	0
Gross deferred income tax liabilities	60,412	40,235
Net deferred income tax assets before valuation allowance	43,747	45,861
Valuation allowance	(28,128)	(25,864)	$ (18,829)	$ (14,498)
Net deferred income tax assets	15,619	19,997
Amounts recognized in the Consolidated Balance Sheets consist of:
Long-term deferred income tax assets included in Deferred income tax assets	17,971	22,146
Long-term deferred income tax liabilities included in Other noncurrent liabilities	(2,352)	(2,149)
Net deferred income tax assets	$ 15,619	$ 19,997