Debt - Schedule of Long-Term Debt (Parenthetical) (Detail)	6 Months Ended
	Jun. 30, 2017
6.50% Bonds, Due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt, interest rate	6.50%
Debt instrument, maturity date	2018
6.75% Notes, Due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt, interest rate	6.75%
Debt instrument, maturity date	2018
5.72% Notes, Due 2019 [Member]
Debt Instrument [Line Items]
Long-term debt, interest rate	5.72%
Debt instrument, maturity date	2019
1.63% Convertible Notes, Due 2019 [Member]
Debt Instrument [Line Items]
Long-term debt, interest rate	1.63%
Debt instrument, maturity date	2019
6.150% Notes, Due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt, interest rate	6.15%
Debt instrument, maturity date	2020
5.40% Notes, Due 2021 [Member]
Debt Instrument [Line Items]
Long-term debt, interest rate	5.40%
Debt instrument, maturity date	2021
5.87% Notes, Due 2022 [Member]
Debt Instrument [Line Items]
Long-term debt, interest rate	5.87%
Debt instrument, maturity date	2022
5.125% Notes, Due 2024 [Member]
Debt Instrument [Line Items]
Long-term debt, interest rate	5.125%
Debt instrument, maturity date	2024
5.90% Notes, Due 2027 [Member]
Debt Instrument [Line Items]
Long-term debt, interest rate	5.90%
Debt instrument, maturity date	2027
6.75% Bonds, Due 2028 [Member]
Debt Instrument [Line Items]
Long-term debt, interest rate	6.75%
Debt instrument, maturity date	2028
5.95% Notes Due 2037 [Member]
Debt Instrument [Line Items]
Long-term debt, interest rate	5.95%
Debt instrument, maturity date	2037
Iowa Finance Authority Loan, Due 2042 [Member]
Debt Instrument [Line Items]
Debt instrument, maturity date	2042