Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2011	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Tax Credit Carryforward [Line Items]
Tolling Payment Payable to AFG		$ 30,496	$ 28,691
Impact of AMT Repeal on Current Taxes		$ 29,581
Congressional Budget Sequestration Haircut		6.60%
Impact of new Tax Law on Deferred Taxes		$ 31,418
Net Impact of New Tax Law		$ 1,886
AMT Installment Payment End Date		Dec. 31, 2021
Tolling Payment Payable to AFG		$ 31,133	28,691
Unrecognized Tax Benefits, Interest on Income Taxes Accrued		0	$ 0	$ 0
NOL allocated amount	$ 3,650,000	3,694,844
AFG [Member]
Tax Credit Carryforward [Line Items]
NOL allocated amount		1,412,639
Percentage Of Notional Federal Tax Liability	50.00%
U. S. Federal Net Operating Tax [Member]
Tax Credit Carryforward [Line Items]
NOL allocated amount		$ 3,694,844
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