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Subsequent Events (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Mar. 31, 2018
Jun. 30, 2018
Subsequent Event [Line Items]      
Common Stock, Dividends, Per Share, Cash Paid [1] $ 1.050 $ 1.050  
2018 ATM, Aggregate Value of Common Stock $ 500   $ 500
Common Stock [Member]      
Subsequent Event [Line Items]      
Dividends Payable, Date to be Paid [1] Jun. 29, 2018 Mar. 30, 2018  
Dividends Payable, Date of Record [1] Jun. 15, 2018 Mar. 16, 2018  
Dividends Payable, Date Declared [1] May 17, 2018 Feb. 21, 2018  
Lightower Acquisition [Member]      
Subsequent Event [Line Items]      
Business Acquisition, Date of Acquisition Agreement     Nov. 01, 2017
Dividend Declared [Member] | Common Stock [Member]      
Subsequent Event [Line Items]      
Dividends Payable, Date to be Paid     Sep. 28, 2018
Dividends Payable, Date of Record     Sep. 14, 2018
Dividends Payable, Date Declared     Aug. 02, 2018
Paid subsequent to year end [Member]      
Subsequent Event [Line Items]      
Common Stock, Dividends, Per Share, Cash Paid $ 1.050    
July 2018 Tower Revenue Notes [Member] | Issuance of debt obligation [Member]      
Subsequent Event [Line Items]      
Debt Instrument, Face Amount $ 1,000   $ 1,000
3.720% July 2018 Tower Revenue Notes [Member] | Issuance of debt obligation [Member]      
Subsequent Event [Line Items]      
Debt Instrument, Face Amount   $ 250  
Debt Instrument, Interest Rate, Stated Percentage   3.70%  
Debt Instrument, Maturity Date   Jul. 01, 2043  
4.241% July 2018 Tower Revenue Notes [Member] | Issuance of debt obligation [Member]      
Subsequent Event [Line Items]      
Debt Instrument, Face Amount   $ 750  
Debt Instrument, Interest Rate, Stated Percentage   4.20%  
Debt Instrument, Maturity Date   Jul. 01, 2048  
2018 Tower Revenue Notes Risk Retention Tranche [Member] | Issuance of debt obligation [Member]      
Subsequent Event [Line Items]      
Debt Instrument, Face Amount   $ 53  
[1] Inclusive of dividends accrued for holders of unvested restricted stock units, which will be paid when and if the restricted stock units vest.