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Debt (Secured Tower Revenue Securities) (Narrative) (Details)										12 Months Ended
	Feb. 16, 2018 USD ($)	Apr. 17, 2017 USD ($)	Jul. 15, 2016 USD ($)	Jul. 07, 2016 USD ($)	Oct. 14, 2015 USD ($)	Oct. 15, 2014 USD ($)	Apr. 18, 2013 USD ($)	Aug. 09, 2012 USD ($)	Apr. 16, 2010 USD ($)	Dec. 31, 2017 USD ($) site	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Long-term debt										$ 9,310,686,000	$ 8,775,583,000
Tower Securities [Member]
Debt Instrument [Line Items]
Aggregate number of tower sites owned by Borrowers \| site										10,442
Property management fee percentage										4.50%
U.S. Treasury rate term										10 years
Interest added to Treasury rate and credit-based spread for non-compliance										5.00%
Debt instrument, additional borrowings										$ 760,000,000
Long-term debt										$ 4,800,000,000
2010 Tower Securities [Member]
Debt Instrument [Line Items]
Deferred financing fees									$ 8,100,000
2010-2C Tower Securities [Member]
Debt Instrument [Line Items]
Debt instrument, face amount									$ 550,000,000
Debt instrument, interest rate, stated percentage									5.101%
Repayment date of debt instrument									Apr. 11, 2017
Debt instrument, maturity date									Apr. 09, 2042
Write-off of deferred financing fees			$ 1,000,000
Repayments of long-term debt				$ 550,000,000
2012 Tower Securities [Member]
Debt Instrument [Line Items]
No prepayment consideration period										12 months
2012-1C Tower Securities [Member]
Debt Instrument [Line Items]
Debt instrument, face amount								$ 610,000,000
Debt instrument, interest rate, stated percentage								2.933%
Repayment date of debt instrument								Dec. 11, 2017
Debt instrument, maturity date								Dec. 09, 2042		Dec. 11, 2017
Deferred financing fees								$ 14,900,000
Write-off of deferred financing fees		$ 2,000,000
Repayments of long-term debt		610,000,000
Long-term debt											607,157,000
2013 Tower Securities [Member]
Debt Instrument [Line Items]
Debt instrument, face amount							$ 1,330,000,000
Deferred financing fees							$ 25,500,000
Debt instrument, weighted average interest rate							3.218%
2013-1C Tower Securities [Member]
Debt Instrument [Line Items]
No prepayment consideration period										12 months
Debt instrument, face amount							$ 425,000,000
Debt instrument, interest rate, stated percentage							2.24%
Repayment date of debt instrument							Apr. 10, 2018
Debt instrument, maturity date							Apr. 09, 2043			Apr. 10, 2018
Long-term debt										$ 424,482,000	422,768,000
2013-2C Tower Securities [Member]
Debt Instrument [Line Items]
No prepayment consideration period										18 months
Debt instrument, face amount							$ 575,000,000
Debt instrument, interest rate, stated percentage							3.722%
Repayment date of debt instrument							Apr. 11, 2023
Debt instrument, maturity date							Apr. 09, 2048			Apr. 11, 2023
Long-term debt										$ 568,609,000	567,545,000
2013-1D Tower Securities [Member]
Debt Instrument [Line Items]
No prepayment consideration period										12 months
Debt instrument, face amount							$ 330,000,000
Debt instrument, interest rate, stated percentage							3.598%
Repayment date of debt instrument							Apr. 10, 2018
Debt instrument, maturity date							Apr. 09, 2043			Apr. 10, 2018
Long-term debt										$ 329,585,000	328,225,000
2014 Tower Securities [Member]
Debt Instrument [Line Items]
Debt instrument, face amount						$ 1,540,000,000
Deferred financing fees						$ 22,500,000
Debt instrument, weighted average interest rate						3.289%
2014-1C Tower Securities [Member]
Debt Instrument [Line Items]
No prepayment consideration period										12 months
Debt instrument, face amount						$ 920,000,000
Debt instrument, interest rate, stated percentage						2.898%
Repayment date of debt instrument						Oct. 08, 2019
Debt instrument, maturity date						Oct. 11, 2044				Oct. 08, 2019
Long-term debt										$ 914,929,000	912,219,000
2014-2C Tower Securities [Member]
Debt Instrument [Line Items]
No prepayment consideration period										18 months
Debt instrument, face amount						$ 620,000,000
Debt instrument, interest rate, stated percentage						3.869%
Repayment date of debt instrument						Oct. 08, 2024
Debt instrument, maturity date						Oct. 08, 2049				Oct. 08, 2024
Long-term debt										$ 613,461,000	612,641,000
2015-1C Tower Securities [Member]
Debt Instrument [Line Items]
No prepayment consideration period										12 months
Debt instrument, face amount					$ 500,000,000
Debt instrument, interest rate, stated percentage					3.156%
Repayment date of debt instrument					Oct. 08, 2020
Debt instrument, maturity date					Oct. 10, 2045					Oct. 08, 2020
Deferred financing fees					$ 11,200,000
Debt service coverage ratio										2
Long-term debt										$ 493,474,000	491,289,000
2016-1C Tower Securities [Member]
Debt Instrument [Line Items]
Debt instrument, face amount				$ 700,000,000
Debt instrument, interest rate, stated percentage				2.877%
Repayment date of debt instrument				Jul. 09, 2021
Debt instrument, maturity date				Jul. 10, 2046						Jul. 09, 2021
Deferred financing fees				$ 9,500,000
Long-term debt										$ 693,118,000	$ 691,322,000
2017-1C Tower Securities [Member]
Debt Instrument [Line Items]
Debt instrument, face amount		$ 760,000,000
Debt instrument, interest rate, stated percentage		3.168%
Repayment date of debt instrument		Apr. 11, 2022
Debt instrument, maturity date		Apr. 09, 2047								Apr. 11, 2022
Deferred financing fees		$ 10,200,000
Long-term debt										$ 751,076,000
2017-1R Tower Securities [Member]
Debt Instrument [Line Items]
Debt instrument, face amount		$ 40,000,000
Debt instrument, interest rate, stated percentage		4.459%
Repayment date of debt instrument		Apr. 11, 2022
Debt instrument, maturity date		Apr. 09, 2047
2017 Tower Securities [Member]
Debt Instrument [Line Items]
Debt instrument, face amount		$ 800,000,000
Debt instrument, unused amount		$ 190,000,000
2013 1-C and 2013 1-D Tower Securities [Member]
Debt Instrument [Line Items]
Long-term debt										$ 755,000,000
Minimum [Member] \| Tower Securities [Member]
Debt Instrument [Line Items]
Additional interest rate for non-compliance										5.00%
Subsequent Event [Member] \| 2018-1C Tower Securities [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 640,000,000
Debt instrument, interest rate, stated percentage	3.448%
Repayment date of debt instrument	Mar. 09, 2023
Debt instrument, maturity date	Mar. 09, 2048
Subsequent Event [Member] \| 2018-1R Tower Securities [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 33,700,000
Debt instrument, interest rate, stated percentage	4.949%
Repayment date of debt instrument	Mar. 09, 2023
Debt instrument, maturity date	Mar. 09, 2048
Excess Cash Flow Reserve [Member] \| Minimum [Member] \| Tower Securities [Member]
Debt Instrument [Line Items]
Debt service coverage ratio										1.30
Amortization Period Prepay [Member] \| Maximum [Member] \| Tower Securities [Member]
Debt Instrument [Line Items]
Debt service coverage ratio										1.15