Income Taxes - Attributes Available for Utilization (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
U.S. federal net operating losses [Member]
Operating Loss Carryforwards [Line Items]
Attribute amount	$ 0.1
Deferred Tax Asset Attributes, First Year of Expiration	Dec. 31, 2033
U.S. state net operating losses [Member]
Operating Loss Carryforwards [Line Items]
Attribute amount	$ 1,326.2
Deferred Tax Asset Attributes, First Year of Expiration	Dec. 31, 2018
U.S. federal tax credits [Member]
Operating Loss Carryforwards [Line Items]
Attribute amount	$ 0.2
Deferred Tax Asset Attributes, First Year of Expiration	Dec. 31, 2021
U.S. state tax credits [Member]
Operating Loss Carryforwards [Line Items]
Attribute amount	$ 5.5
Deferred Tax Asset Attributes, First Year of Expiration	Dec. 31, 2020
Foreign net operating losses [Member]
Operating Loss Carryforwards [Line Items]
Attribute amount	$ 350.0
Deferred Tax Asset Attributes, First Year of Expiration	Dec. 31, 2018