Income Taxes - Asset/Liability (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Net losses and credit carry-forwards	$ 47,489	$ 60,329
Contract liabilities	110,653	120,583
Investment in affiliates	1,078	4,626
Employee benefits	28,613	28,468
Accrued expenses and loss reserves	16,118	13,512
Operating lease liabilities	27,311	25,748
Unrealized gains and losses	23,705	14,371
Other	16,751	13,640
Less: valuation allowance	(40,656)	(49,863)
Total deferred tax assets	231,062	231,414
Deferred tax liabilities:
Depreciable and amortizable assets	305,147	293,151
Operating lease assets	17,768	15,891
Investment in affiliates	0	15,360
Deferred tax liabilities	322,915	324,402
Net deferred tax liabilities	$ (91,853)	$ (92,988)