Supplemental Information to Consolidated Statements of Cash Flows - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	2 Months Ended	3 Months Ended		12 Months Ended
	Jun. 30, 2017	Feb. 28, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
4.25% Convertible Senior Notes (due 2018)
Cash Flow Supplemental Disclosures [Line Items]
Debt instrument shares common stock issued upon conversion (in shares)				2,900,000
Common Stock
Cash Flow Supplemental Disclosures [Line Items]
Restricted stock surrendered, (in shares)						9,000	25,000
Common Stock | 4.25% Convertible Senior Notes (due 2018)
Cash Flow Supplemental Disclosures [Line Items]
Debt instrument shares common stock issued upon conversion (in shares)	112,400	2,865,000	112,400			3,000,000
Common stock shares issued upon conversion of debt instrument, value	$ 0.4	$ 15.1				$ 15.5
Restricted Stock
Cash Flow Supplemental Disclosures [Line Items]
Restricted stock surrendered, (in shares)					42,346	29,689	50,719
Restricted stock surrendered, in values					$ 0.1	$ 0.1	$ 1.5
Tax benefit (deficiency) related to restricted stock compensation							$ 0.5
Employees - including an executive officer
Cash Flow Supplemental Disclosures [Line Items]
Restricted stock surrendered, (in shares)					42,346	29,689	50,719
Restricted stock surrendered, in values							$ 1.5
Employees - including an executive officer | Maximum
Cash Flow Supplemental Disclosures [Line Items]
Restricted stock surrendered, in values					$ 0.0	$ 0.1