Income Taxes - Tax Effect of Significant Items Comprising the Net Deferred Tax Asset (Liability) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax asset:
Benefits from net operating loss carryforwards	$ 2,518	$ 3,405
Long-term incentive compensation plans	22,342	21,204
Investment basis differences	3,606	5,967
Depreciation and amortization	1,248	2,214
Other, primarily accrued expenses deductible when paid	3,903	3,601
Deferred tax assets, gross	33,617	36,391
Less: valuation allowance	(490)	(497)
Deferred tax asset	33,127	35,894
Deferred tax liability:
Intangible assets	6,852	6,286
Investment in foreign subsidiaries	1,653	0
Other	1,758	1,007
Deferred tax liability	10,263	7,293
Net deferred tax asset	$ 22,864	$ 28,601