Debt (Secured Tower Revenue Securities Series 2010) (Details) (USD $) In Millions, unless otherwise specified	0 Months Ended	3 Months Ended
	Apr. 16, 2010	Mar. 31, 2012
2010-1 Tower Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Proceeds from issuance of 2010 Tower Securities, net of fees paid	$ 680.0
Securities, annual interest rate		4.254%
Tower Revenue Securities anticipated repayment date		April 16, 2015
Tower Revenue Securities final maturity date		April 16, 2040
2010-2 Tower Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Proceeds from issuance of 2010 Tower Securities, net of fees paid	$ 550.0
Securities, annual interest rate		5.101%
Tower Revenue Securities anticipated repayment date		April 16, 2017
Tower Revenue Securities final maturity date		April 16, 2042
Tower Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted average annual fixed coupon interest rate		4.70%