Debt and Interest Expense - Long-Term Debt (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2014
Debt Instrument [Line Items]
Weighted average interest rate	6.40%
Other Fixed Rate Notes 12.9% [Member]
Debt Instrument [Line Items]
Weighted average interest rate	12.90%
Project Lease Financing 5.1% [Member]
Debt Instrument [Line Items]
Weighted average interest rate	5.10%
7.0% Due 2014 [Member]
Debt Instrument [Line Items]
Coupon rate	7.00%
Maturity year	2014
1.3% Due 2017 [Member]
Debt Instrument [Line Items]
Coupon rate	1.30%
Maturity year	2017
8.1% Due 2019 [Member]
Debt Instrument [Line Items]
Coupon rate	8.10%
Maturity year	2019
3.5% Due 2024 [Member]
Debt Instrument [Line Items]
Coupon rate	3.50%
Maturity year	2024
7.9% Due 2029 [Member]
Debt Instrument [Line Items]
Coupon rate	7.90%
Maturity year	2029
7.3% Due 2031 [Member]
Debt Instrument [Line Items]
Coupon rate	7.30%
Maturity year	2031
7.1% Due 2033 [Member]
Debt Instrument [Line Items]
Coupon rate	7.10%
Maturity year	2033
6.0% Due 2040 [Member]
Debt Instrument [Line Items]
Coupon rate	6.00%
Maturity year	2040
5.6% Due 2041 [Member]
Debt Instrument [Line Items]
Coupon rate	5.60%
Maturity year	2041