Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018
Class of Stock [Line Items]
Preferred stock, shares authorized			75,000,000
Preferred stock, par value			$ 0.001
Common stock, shares authorized	300,000,000		400,000,000
Common stock, par value	$ 0.001	$ 0.001	$ 0.001
Preferred stock, shares outstanding			0
Preferred stock, shares issued			0
Number of Shares Sold, Equity Distribution Agreement			0
Common Stock Shares Issued for Debt Exchange			0
Shares, Issued	110,363,539		212,477,101
Equity Offering [Member]
Class of Stock [Line Items]
Partial Exercise Of Option To Upsize Purchase Of Common Stock	2,205,529
Shares, Issued	23,205,529	15,525,000	0
Shares Issued, Price Per Share	$ 5.00	$ 7.40
Option To Upsize Purchase Of Common Stock	3,150,000	2,025,000
Proceeds from Issuance or Sale of Equity	$ 110.8	$ 109.7
Seven Point Six Two Five Percentage Senior Notes [Member]
Class of Stock [Line Items]
Common Stock Shares Issued for Debt Exchange		10,000,000
Debt, stated interest rate			7.625%
Amount of debt converted		$ 84.7
Seven Point Six Two Five Percentage Senior Notes Due Two Thousand And Nineteen [Member]
Class of Stock [Line Items]
Debt, stated interest rate		7.625%
Seven Point Zero Percentage Senior Notes [Member]
Class of Stock [Line Items]
Common Stock Shares Issued for Debt Exchange	10,863,000
Debt, stated interest rate			7.00%
Amount of debt converted	$ 50.0
Merger [Member]
Class of Stock [Line Items]
Shares, Issued	0	0	100,000,000