Summary of Additional Interest Expense Recognized (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule Of Interest Expenses [Line Items]
Total	$ 29,796	$ 28,404	$ 27,082
Convertible Notes due 2018
Schedule Of Interest Expenses [Line Items]
Amortization of issuance costs	1,931	1,921	1,910
Accretion of discount on convertible notes	14,337	13,633	12,963
Convertible Notes due 2020
Schedule Of Interest Expenses [Line Items]
Amortization of issuance costs	1,288	1,283	1,279
Accretion of discount on convertible notes	$ 12,240	$ 11,567	$ 10,930