Income Taxes - Narrative (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jun. 30, 2016
Income Taxes [Line Items]
Net operating loss carryovers	$ 1,500,000,000
Tax Cuts and Jobs Act of 2017, provisional income tax benefit	700,000
Tax Cuts and Jobs Act of 2017, change in tax rate, deferred tax asset, provisional income tax expense (benefit)		$ 207,700,000
Tax Cuts and Jobs Act of 2017, provisional income tax benefit		800,000
Income (loss) before income taxes, foreign	0	$ 0	$ 1,400,000
Alternative minimum tax credit carryovers	$ 1,400,000
Minimum
Income Taxes [Line Items]
Net operating loss carryforwards expiration year	2027
Maximum
Income Taxes [Line Items]
Net operating loss carryforwards expiration year	2036
5% Convertible Senior Notes due 2021
Income Taxes [Line Items]
Deferred tax asset, increase (decrease)			$ 8,600,000
Debt instrument, interest rate	5.00%			5.00%
Convertible Debt | 5% Convertible Senior Notes due 2021
Income Taxes [Line Items]
Debt instrument, interest rate	5.00%		5.00%