Fair Value Measurements - Components of Long-Term Debt and Methods Used in Estimating Fair Values (Detail)	9 Months Ended
	Sep. 30, 2018
3.875% Convertible Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument, maturity date	2019
Valuation Methodology	Quoted price
4.25% Convertible Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument, maturity date	2037
Valuation Methodology	Quoted price
3.95% Promissory Note [Member]
Debt Instrument [Line Items]
Debt instrument, maturity date	2020
Valuation Methodology	Discounted cash flow method/Level 3 inputs
4% Promissory Note [Member]
Debt Instrument [Line Items]
Debt instrument, maturity date	2031
Valuation Methodology	Discounted cash flow method/Level 3 inputs
3.75% Callable Promissory Note [Member]
Debt Instrument [Line Items]
Debt instrument, maturity date	2036
Valuation Methodology	Discounted cash flow method/Level 3 inputs
4.55% Promissory Note [Member]
Debt Instrument [Line Items]
Debt instrument, maturity date	2036
Valuation Methodology	Discounted cash flow method/Level 3 inputs