Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2018	Dec. 31, 2016	Sep. 30, 2011
Tax Credit Carryforward [Line Items]
Deferred Tax Liabilities, Net	$ 40,130	$ 33,659
Tolling Payment Payable to AFG	44,381	30,496
Impact of AMT Repeal on Current Taxes		29,581
Impact of new Tax Law on Deferred Taxes		31,418
Net Impact of New Tax Law		1,886
Tolling Payment Payable to AFG	13,885			$ 28,691
Unrecognized Tax Benefits, Interest on Income Taxes Accrued	0	$ 0		$ 0
NOL allocated amount	3,423,704				$ 3,650,000
AFG [Member]
Tax Credit Carryforward [Line Items]
NOL allocated amount	$ 1,260,583
U. S. Federal Net Operating Tax [Member]
Tax Credit Carryforward [Line Items]
NOL allocated amount			$ 3,423,704
Minimum [Member]
Tax Credit Carryforward [Line Items]
Operating Loss Carryforwards, Expiration Date	Dec. 31, 2029
Maximum [Member]
Tax Credit Carryforward [Line Items]
Operating Loss Carryforwards, Expiration Date	Dec. 31, 2032