Income Taxes (Asset/Liability) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net losses and credit carryforwards	$ 68,744	$ 90,773
Deferred revenue	114,586	149,022
Investment in affiliates	4,375	14,501
Employee benefits	31,944	47,823
Accrued expenses and loss reserves	28,043	38,024
Other	12,296	20,692
Less: valuation allowance	(45,166)	(44,879)
Total deferred tax assets	214,822	315,956
Deferred tax liabilities:
Depreciable and amortizable assets	299,578	415,879
Investment in affiliates	17,449	18,624
Deferred tax liabilities	317,027	434,503
Net deferred tax liabilities	$ (102,205)	$ (118,547)