Income Taxes - Tax Effect of Significant Items Comprising the Net Deferred Tax Asset (Liability) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax asset:
Benefits from net operating loss carryforwards	$ 3,405	$ 4,441
Long-term incentive compensation plans	21,204	25,263
Investment basis differences/net unrealized losses	6,079	2,750
Depreciation and amortization	2,026	2,222
Other, primarily accrued expenses deductible when paid	3,378	3,588
Deferred tax assets, gross	36,092	38,264
Less: valuation allowance	(497)	(462)
Deferred tax asset	35,595	37,802
Deferred tax liability:
Intangible assets	6,103	6,302
Other	891	1,960
Deferred tax liability	6,994	8,262
Net deferred tax asset	$ 28,601	$ 29,540