Summary of Debt (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
5.60% notes due 2012
Debt Instrument [Line Items]
Interest rate	5.60%	5.60%	5.60%
Maturity year	2012	2012	2012
Decrease in unamortized discounts			$ 36
6.30% notes due 2013
Debt Instrument [Line Items]
Interest rate	6.30%	6.30%	6.30%
Maturity year	2013	2013	2013
Decrease in unamortized discounts	14	30	77
Effective interest rate	7.48%	7.48%	7.48%
10.125% notes due 2015
Debt Instrument [Line Items]
Interest rate	10.125%	10.125%	10.125%
Maturity year	2015	2015	2015
Decrease in unamortized discounts	246	265	320
Effective interest rate	9.59%	9.59%	9.59%
Increase (decrease) in unamortized deferred gain realized	2,766	2,983	3,604
6.50% notes due 2016
Debt Instrument [Line Items]
Interest rate	6.50%	6.50%	6.50%
Maturity year	2016	2016	2016
Effective interest rate	6.02%	6.02%	6.02%
Increase (decrease) in unamortized deferred gain realized	14,221	15,060	17,503
6.40% notes due 2017
Debt Instrument [Line Items]
Interest rate	6.40%	6.40%	6.40%
Maturity year	2017	2017	2017
Decrease in unamortized discounts	108	112	126
Effective interest rate	7.41%	7.41%	7.41%
7.00% notes due 2018
Debt Instrument [Line Items]
Interest rate	7.00%	7.00%	7.00%
Maturity year	2018	2018	2018
Decrease in unamortized discounts	259	269	298
Effective interest rate	7.87%	7.87%	7.87%
10.375% notes due 2018
Debt Instrument [Line Items]
Interest rate	10.375%	10.375%	10.375%
Maturity year	2018	2018	2018
Decrease in unamortized discounts	1,284	1,324	1,438
Effective interest rate	10.62%	10.62%	10.62%
7.50% notes due 2021
Debt Instrument [Line Items]
Interest rate	7.50%	7.50%	7.50%
Maturity year	2021	2021	2021
Effective interest rate	7.75%
7.15% notes due 2037
Debt Instrument [Line Items]
Interest rate	7.15%	7.15%	7.15%
Maturity year	2037	2037	2037
Decrease in unamortized discounts	633	635	$ 641
Effective interest rate	8.05%	8.05%	8.05%