Debt Facilities - Components of Total Interest Expense Recognized Related to Convertible Subordinated Notes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Interest charges incurred	$ 501,323	$ 405,494	$ 309,998
Effective interest rate of the liability component (percent)	7.86%
Convertible Subordinated Debt | 4.75% convertible subordinated notes
Debt Instrument [Line Items]
Contractual interest expense		3,267
Amortization of debt issuance costs		186
Amortization of debt discount		3,775
Interest charges incurred		$ 7,228
Effective interest rate of the liability component (percent)		10.48%