Income Taxes - Summary of Net Operating Loss and Tax Credit Carryforwards (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
State [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards, amount	$ 299,631
Tax credit carryforward, amount	$ 82,617
State [Member] | Earliest Tax Year [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards, expiration year	2021
Tax credit carryforward, expiration year	2019
State [Member] | Latest Tax Year [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards, expiration year	2036
Tax credit carryforward, expiration	indefinite
Federal [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards, amount	$ 409,273
Tax credit carryforward, amount	$ 89,218
Federal [Member] | Earliest Tax Year [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards, expiration year	2029
Tax credit carryforward, expiration year	2030
Federal [Member] | Latest Tax Year [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards, expiration year	2036
Tax credit carryforward, expiration year	2037