Debt - Carrying Values of Debt (Parenthetical) (Detail)	Mar. 31, 2013	May 16, 2008
8.25% Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage	8.25%
5.625% Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage	5.625%
5.75% Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage	5.75%
4.254% Secured Tower Revenue Securities Series 2010-1 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage	4.254%
5.101% Secured Tower Revenue Securities Series 2010-2 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage	5.101%
2.933% Secured Tower Revenue Securities Series 2012-1 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage	2.933%
1.875% Convertible Senior Notes due 2013 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage	1.875%	1.875%
4.0% Convertible Senior Notes due 2014 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage	4.00%