American Tower (AMT) 10-K 28 Feb 18 2017 Q4 Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2017	Feb. 20, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Entity Registrant Name	AMERICAN TOWER CORP /MA/
Entity Central Index Key	1,053,507
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Document Type	10-K
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus (Q1,Q2,Q3,FY)	Q4
Amendment Flag	false
Entity Common Stock, Shares Outstanding		440,851,771
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 56.3

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
CURRENT ASSETS:
Cash and cash equivalents	$ 802.1	$ 787.2
Restricted cash	152.8	149.3
Short-term investments	1	4
Accounts receivable, net	513.6	308.4
Prepaid and other current assets	568.6	441
Total current assets	2,038.1	1,689.9
PROPERTY AND EQUIPMENT, net	11,101	10,517.3
GOODWILL	5,638.4	5,070.7
OTHER INTANGIBLE ASSETS, net	11,783.3	11,274.6
DEFERRED TAX ASSET	204.4	195.7
DEFERRED RENT ASSET	1,499	1,289.5
NOTES RECEIVABLE AND OTHER NON-CURRENT ASSETS	950.1	841.5
TOTAL	33,214.3	30,879.2
CURRENT LIABILITIES:
Accounts payable	142.9	118.7
Accrued expenses	854.3	620.5
Distributions payable	304.4	250.6
Accrued interest	166.9	157.3
Current portion of long-term obligations	774.8	238.8
Unearned revenue	268.8	245.4
Total current liabilities	2,512.1	1,631.3
LONG-TERM OBLIGATIONS	19,430.3	18,294.7
ASSET RETIREMENT OBLIGATIONS	1,175.3	965.5
DEFERRED TAX LIABILITY	898.1	777.6
OTHER NON-CURRENT LIABILITIES	1,244.2	1,142.6
Total liabilities	25,260	22,811.7
COMMITMENTS AND CONTINGENCIES
REDEEMABLE NONCONTROLLING INTERESTS	1,126.2	1,091.3
EQUITY (shares in thousands):
Common stock: $.01 par value; 1,000,000 shares authorized; 437,729 and 429,913 shares issued; and 428,820 and 427,103 shares outstanding, respectively	4.4	4.3
Additional paid-in capital	10,247.5	10,043.5
Distributions in excess of earnings	(1,058.1)	(1,077)
Accumulated other comprehensive loss	(1,978.3)	(1,999.3)
Treasury stock (8,909 and 2,810 shares at cost, respectively)	(974)	(207.7)
Total American Tower Corporation equity	6,241.5	6,763.9
Noncontrolling interests	586.6	212.3
Total equity	6,828.1	6,976.2
TOTAL	33,214.3	30,879.2
Convertible Preferred Stock Subject to Mandatory Redemption | 5.25%, Series A, 6,000 shares issued, 0 and 6,000 shares outstanding; aggregate liquidation value of $0.0 and $0.6, respectively
EQUITY (shares in thousands):
Preferred stock: $.01 par value; 20,000 shares authorized;	0	0.1
Convertible Preferred Stock Subject to Mandatory Redemption | 5.50%, Series B, 1,375 shares issued, 1,375 shares outstanding; aggregate liquidation value of $1.4
EQUITY (shares in thousands):
Preferred stock: $.01 par value; 20,000 shares authorized;	0	0
Preferred stock | Convertible Preferred Stock Subject to Mandatory Redemption | 5.25%, Series A, 6,000 shares issued, 0 and 6,000 shares outstanding; aggregate liquidation value of $0.0 and $0.6, respectively
EQUITY (shares in thousands):
Total equity	0	0.1
Preferred stock | Convertible Preferred Stock Subject to Mandatory Redemption | 5.50%, Series B, 1,375 shares issued, 1,375 shares outstanding; aggregate liquidation value of $1.4
EQUITY (shares in thousands):
Total equity	$ 0	$ 0

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	1,000,000,000	1,000,000,000
Common stock, shares issued	437,729,000	429,913,000
Common stock, shares outstanding	428,820,000	427,103,000
Treasury stock, shares	8,909,000,000	2,810,000
Series A Preferred Stock | Convertible Preferred Stock Subject to Mandatory Redemption
Class of Stock [Line Items]
Preferred stock, dividend rate, percentage	5.25%	5.25%
Preferred stock, shares issued	6,000,000	6,000,000
Preferred stock, shares outstanding	0	6,000,000
Preferred stock, aggregate liquidation value	$ 0	$ 600
Series B Preferred Stock | Convertible Preferred Stock Subject to Mandatory Redemption
Class of Stock [Line Items]
Preferred stock, dividend rate, percentage	5.50%
Preferred stock, shares issued	1,375,000
Preferred stock, shares outstanding	1,375,000
Preferred stock, aggregate liquidation value	$ 1,400
Preferred stock | Series A Preferred Stock | Convertible Preferred Stock Subject to Mandatory Redemption
Class of Stock [Line Items]
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized	20,000,000	20,000,000
Preferred stock | Series B Preferred Stock | Convertible Preferred Stock Subject to Mandatory Redemption
Class of Stock [Line Items]
Preferred stock, par value (in dollars per share)	$ 0.01

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
REVENUES:
Property	$ 6,565.9	$ 5,713.1	$ 4,680.4
Services	98	72.6	91.1
Total operating revenues	6,663.9	5,785.7	4,771.5
OPERATING EXPENSES:
Property (including stock-based compensation expense of $2.1, $1.7 and $1.6, respectively)	2,022	1,762.7	1,275.4
Services (including stock-based compensation expense of $0.8, $0.7 and $0.4, respectively)	34.6	27.7	33.4
Depreciation, amortization and accretion	1,715.9	1,525.6	1,285.3
Selling, general, administrative and development expense (including stock-based compensation expense of $105.6, $87.5 and $88.5, respectively)	637	543.4	497.8
Other operating expenses	256	73.3	66.8
Total operating expenses	4,665.5	3,932.7	3,158.7
OPERATING INCOME	1,998.4	1,853	1,612.8
OTHER INCOME (EXPENSE):
(Loss) gain on retirement of long-term obligations	(70.2)	1.2	(79.6)
Other income (expense) (including unrealized foreign currency gains (losses) of $26.5, ($23.4), and ($71.5), respectively)	31.3	(47.7)	(135)
Total other expense	(742.3)	(727.1)	(782.8)
INCOME FROM CONTINUING OPERATIONS BEFORE INCOME TAXES	1,256.1	1,125.9	830
Income tax provision	(30.7)	(155.5)	(158)
NET INCOME	1,225.4	970.4	672
Net loss (income) attributable to noncontrolling interests	13.5	(14)	13.1
NET INCOME ATTRIBUTABLE TO AMERICAN TOWER CORPORATION STOCKHOLDERS	1,238.9	956.4	685.1
Dividends on preferred stock	(87.4)	(107.1)	(90.2)
NET INCOME ATTRIBUTABLE TO AMERICAN TOWER CORPORATION COMMON STOCKHOLDERS	$ 1,151.5	$ 849.3	$ 594.9
NET INCOME PER COMMON SHARE AMOUNTS:
Basic net income attributable to American Tower Corporation common stockholders (in dollars per share)	$ 2.69	$ 2	$ 1.42
Diluted net income attributable to American Tower Corporation common stockholders (in dollars per share)	$ 2.67	$ 1.98	$ 1.41
WEIGHTED AVERAGE COMMON SHARES OUTSTANDING:
BASIC (in shares)	428,181	425,143	418,907
DILUTED (in shares)	431,688	429,283	423,015
TV Azteca
OTHER INCOME (EXPENSE):
Interest income, TV Azteca, net of interest expense of $1.2, $1.2 and $0.8, respectively	$ 10.8	$ 10.9	$ 11.2
Interest expense	(1.2)	(1.2)	(0.8)
Excluding TV Azteca
OTHER INCOME (EXPENSE):
Interest income	35.4	25.6	16.5
Interest expense	$ (749.6)	$ (717.1)	$ (595.9)

CONSOLIDATED STATEMENTS OF OPE5

CONSOLIDATED STATEMENTS OF OPERATIONS (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock-based compensation expense	$ 108.5	$ 89.9	$ 90.5
TV Azteca
Interest expense	1.2	1.2	0.8
Property
Stock-based compensation expense	2.1	1.7	1.6
Services
Stock-based compensation expense	0.8	0.7	0.4
Selling General Administrative And Development Expense
Stock-based compensation expense	105.6	87.5	88.5
Other Expense
Foreign currency gain (loss)	$ 26.5	$ (23.4)	$ (71.5)

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 1,225.4	$ 970.4	$ 672
Other comprehensive (loss) income:
Changes in fair value of cash flow hedges, net of tax expense of $0, $0 and $0.1, respectively	(0.4)	(0.4)	0.9
Reclassification of unrealized losses on cash flow hedges to net income, net of tax expense of $0, $0 and $0.1, respectively	(0.1)	(0.3)	2.4
Foreign currency translation adjustments, net of tax expense (benefit) of $1.0, $3.8 million, and $(24.9), respectively	144.4	(202.9)	(1,078.9)
Other comprehensive income (loss)	143.9	(203.6)	(1,075.6)
Comprehensive income (loss)	1,369.3	766.8	(403.6)
Comprehensive (income) loss attributable to noncontrolling interest	(109.4)	18.2	45.9
Comprehensive income (loss) attributable to American Tower Corporation stockholders	$ 1,259.9	$ 785	$ (357.7)

CONSOLIDATED STATEMENTS OF COM7

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net change in fair value of cash flow hedges, tax	$ 0	$ 0	$ 0.1
Reclassification of unrealized losses on cash flow hedges to net income, tax	0	0	0.1
Foreign currency translation adjustments, tax	$ 1	$ 3.8	$ (24.9)

CONSOLIDATED STATEMENTS OF EQUI

CONSOLIDATED STATEMENTS OF EQUITY - USD ($) shares in Thousands, $ in Millions	Total	Common Stock	Treasury Stock	Additional Paid-in Capital	Accumulated Other Comprehensive (Loss) Income	Earnings (Distributions) in Excess of Distributions (Earnings)	Noncontrolling Interest	Series A Preferred StockConvertible Preferred Stock Subject to Mandatory RedemptionPreferred Stock	Series B Preferred Stock	Series B Preferred StockAdditional Paid-in Capital	Series B Preferred StockConvertible Preferred Stock Subject to Mandatory RedemptionPreferred Stock
BALANCE (shares) at Dec. 31, 2014		399,509	(2,810)					6,000			0
BALANCE at Dec. 31, 2014	$ 4,053.4	$ 4	$ (207.7)	$ 5,788.8	$ (794.2)	$ (837.3)	$ 99.7	$ 0.1			$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock based compensation related activity (shares)		1,253
Stock-based compensation related activity	117.2	$ 0		117.2
Issuance of common stock - stock purchase plan (shares)		83
Issuance of common stock—stock purchase plan	6.6	$ 0		6.6
Issuance of stock (shares)		25,850									1,375
Issuance of stock	2,440.4	$ 0.3		2,440.1					$ 1,337.9	$ 1,337.9	$ 0
Changes in fair value of cash flow hedges, net of tax	0.9				0.9		0
Reclassification of unrealized gains on cash flow hedges to net income, net of tax	2.4				2.5		(0.1)
Foreign currency translation adjustment, net of tax	(1,078.9)				(1,046.2)		(32.7)
Contributions from noncontrolling interest	8.1						8.1
Distributions to noncontrolling interest	(0.9)						(0.9)
Common stock distributions declared	(769.5)					(769.5)
Preferred stock dividends declared	(76.8)					(76.8)
Net income	672					685.1	(13.1)
BALANCE (shares) at Dec. 31, 2015		426,695	(2,810)					6,000			1,375
BALANCE at Dec. 31, 2015	6,712.8	$ 4.3	$ (207.7)	9,690.6	(1,837)	(998.5)	61	$ 0.1			$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock based compensation related activity (shares)		1,959
Stock-based compensation related activity	155.1	$ 0		155.1
Issuance of common stock - stock purchase plan (shares)		88
Issuance of common stock—stock purchase plan	7.5	$ 0		7.5
Issuance of stock (shares)		1,171
Issuance of stock	120.8	$ 0		120.8
Changes in fair value of cash flow hedges, net of tax	(0.4)				(0.4)		0
Reclassification of unrealized gains on cash flow hedges to net income, net of tax	(0.3)				(0.3)		0
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax, Less Amount Reclassified to Additional Paid in Capital	(179.4)				(170.7)		(8.7)
Foreign currency translation adjustment, net of tax	(202.9)
Contributions from noncontrolling interest	239.5			69.5	9.1		160.9
Distributions to noncontrolling interest	(1)						(1)
Common stock distributions declared	(927.8)					(927.8)
Preferred stock dividends declared	(107.1)					(107.1)
Net income	956.5					956.4	0.1
BALANCE (shares) at Dec. 31, 2016		429,913	(2,810)					6,000			1,375
BALANCE at Dec. 31, 2016	6,976.2	$ 4.3	$ (207.7)	10,043.5	(1,999.3)	(1,077)	212.3	$ 0.1			$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock based compensation related activity (shares)		2,121
Stock-based compensation related activity	195	$ 0		195
Issuance of common stock - stock purchase plan (shares)		93
Issuance of common stock—stock purchase plan	9	$ 0		9
Conversion of preferred stock (shares)		5,602						(6,000)			0
Conversion of preferred stock	0	$ 0.1		0				$ (0.1)			$ 0
Treasury stock activity (in shares)			(6,099)
Treasury stock activity	(766.3)		$ (766.3)
Changes in fair value of cash flow hedges, net of tax	(0.4)				(0.4)
Reclassification of unrealized gains on cash flow hedges to net income, net of tax	(0.1)				(0.1)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax, Less Amount Reclassified to Additional Paid in Capital	76.1				21.5		54.6
Foreign currency translation adjustment, net of tax	144.4
Contributions from noncontrolling interest	314.1						314.1
Distributions to noncontrolling interest	(14.3)						(14.3)
Common stock distributions declared	(1,128.6)					(1,128.6)
Preferred stock dividends declared	(91.4)					(91.4)
Net income	1,258.8					1,238.9	19.9
BALANCE (shares) at Dec. 31, 2017		437,729	(8,909)					0			1,375
BALANCE at Dec. 31, 2017	$ 6,828.1	$ 4.4	$ (974)	$ 10,247.5	$ (1,978.3)	$ (1,058.1)	$ 586.6	$ 0			$ 0

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CASH FLOWS FROM OPERATING ACTIVITIES
Net income	$ 1,225.4	$ 970.4	$ 672
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation, amortization and accretion	1,715.9	1,525.6	1,285.3
Stock-based compensation expense	108.5	89.9	90.5
(Gain) loss on investments, unrealized foreign currency loss and other non-cash expense	(18)	127.4	146.2
Impairments, net loss on sale of long-lived assets, non-cash restructuring and merger related expenses	242.4	50.7	29.9
Loss (gain) on early retirement of long-term obligations	70.2	(1.2)	79.8
Amortization of deferred financing costs, debt discounts and premiums and other non-cash interest	20	17.7	6.9
Deferred income taxes	(86.6)	27	7.8
Changes in assets and liabilities, net of acquisitions:
Accounts receivable	(191.1)	11.4	(56.3)
Prepaid and other assets	(179.9)	(80)	(91.1)
Deferred rent asset	(194.4)	(131.7)	(155)
Accounts payable and accrued expenses	95.8	(42.9)	95.9
Accrued interest	9.2	34.4	(15.6)
Unearned revenue	59.3	16.6	12.9
Deferred rent liability	62.3	67.8	56.1
Other non-current liabilities	(13.4)	18.6	1.6
Cash provided by operating activities	2,925.6	2,701.7	2,166.9
CASH FLOWS FROM INVESTING ACTIVITIES
Payments for purchase of property and equipment and construction activities	(803.6)	(682.5)	(728.8)
Payments for acquisitions, net of cash acquired	(2,007)	(1,411.3)	(1,961.1)
Payment for Verizon transaction	0	(4.7)	(5,059.5)
Proceeds from sales of short-term investments and other non-current assets	14.7	13.1	1,032.3
Payments for short-term investments	0	(0.8)	(1,022.8)
Deposits and other	(5)	(16.1)	(1.8)
Cash used for investing activities	(2,800.9)	(2,102.3)	(7,741.7)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from short-term borrowings, net	0	0	9
Borrowings under credit facilities	5,359.4	2,446.8	6,126.6
Proceeds from issuance of senior notes, net	2,674	3,236.4	1,492.3
Proceeds from term loan	0	0	500
Proceeds from other borrowings	0	0	54.5
Proceeds from issuance of securities in securitization transaction	0	0	875
Repayments of notes payable, credit facilities, term loan, senior notes and capital leases	(6,484.4)	(5,093.7)	(6,393.4)
Contributions from noncontrolling interest holders, net	264.3	238.5	7.2
Purchases of common stock	(766.3)	0	0
Proceeds from stock options and stock purchase plan	119.7	92.5	50.7
Distributions paid on common stock	(1,073)	(886.1)	(710.9)
Distributions paid on preferred stock	(91.4)	(107.1)	(84.6)
Proceeds from the issuance of common stock, net	0	0	2,440.3
Proceeds from the issuance of preferred stock, net	0	0	1,337.9
Payment for early retirement of long-term obligations	(75.3)	(0.1)	(85.7)
Deferred financing costs and other financing activities	(40)	(26.5)	(25.8)
Cash (used for) provided by financing activities	(113)	(99.3)	5,593.1
Net effect of changes in foreign currency exchange rates on cash and cash equivalents, and restricted cash	6.7	(26.5)	(29.1)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS, AND RESTRICTED CASH	18.4	473.6	(10.8)
CASH AND CASH EQUIVALENTS, AND RESTRICTED CASH, BEGINNING OF YEAR	936.5	462.9	473.7
CASH AND CASH EQUIVALENTS, AND RESTRICTED CASH, END OF YEAR	$ 954.9	$ 936.5	$ 462.9

BUSINESS AND SUMMARY OF SIGNIFI

BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	12 Months Ended
	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Business —American Tower Corporation (together with its subsidiaries, “ATC” or the “Company”) is one of the largest global real estate investment trusts and a leading independent owner, operator and developer of multitenant communications real estate. The Company’s primary business is the leasing of space on communications sites to wireless service providers, radio and television broadcast companies, wireless data providers, government agencies and municipalities and tenants in a number of other industries. The Company refers to this business as its property operations. Additionally, the Company offers tower-related services in the United States, which the Company refers to as its services operations. These services include site acquisition, zoning and permitting and structural analysis, which primarily support the Company’s site leasing business, including the addition of new tenants and equipment on its sites. The Company’s portfolio primarily consists of towers that it owns and towers that it operates pursuant to long-term lease arrangements, as well as distributed antenna system (“DAS”) networks, which provide seamless coverage solutions in certain in-building and certain outdoor wireless environments. In addition to the communications sites in its portfolio, the Company manages rooftop and tower sites for property owners under various contractual arrangements. The Company also holds other telecommunications infrastructure, including fiber, concrete poles and other assets, and property interests that it leases to communications service providers and third-party tower operators. American Tower Corporation is a holding company that conducts its operations through its directly and indirectly owned subsidiaries and its joint ventures. ATC’s principal domestic operating subsidiaries are American Towers LLC and SpectraSite Communications, LLC. ATC conducts its international operations primarily through its subsidiary, American Tower International, Inc., which in turn conducts operations through its various international holding and operating subsidiaries and joint ventures. The Company operates as a real estate investment trust for U.S. federal income tax purposes (“REIT”). Accordingly, the Company generally is not subject to U.S. federal income taxes on income generated by its REIT operations, including the income derived from leasing space on its towers, as it receives a dividends paid deduction for distributions to stockholders that generally offsets its REIT income and gains. However, the Company remains obligated to pay U.S. federal income taxes on earnings from its domestic taxable REIT subsidiaries (“TRSs”). In addition, the Company’s international assets and operations, regardless of their classification for U.S. tax purposes, continue to be subject to taxation in the foreign jurisdictions where those assets are held or those operations are conducted. The use of TRSs enables the Company to continue to engage in certain businesses while complying with REIT qualification requirements. The Company may, from time to time, change the election of previously designated TRSs to be included as part of the REIT. As of December 31, 2017 , the Company’s REIT qualified businesses included its U.S. tower leasing business, most of its operations in Costa Rica and Mexico, a majority of its operations in Germany and a majority of its indoor DAS networks business and services segment. As of January 2018, the Company’s operations in Nigeria became part of the REIT. Principles of Consolidation and Basis of Presentation —The accompanying consolidated and condensed consolidated financial statements include the accounts of the Company and those entities in which it has a controlling interest. Investments in entities that the Company does not control are accounted for using the equity or cost method, depending upon the Company’s ability to exercise significant influence over operating and financial policies. All intercompany accounts and transactions have been eliminated. As of December 31, 2017, the Company holds (i) a 51% controlling interest, and MTN Group Limited holds a 49% noncontrolling interest, in each of two joint ventures, one in Ghana and one in Uganda, (ii) a 51% controlling interest, and PGGM holds a 49% noncontrolling interest, in a joint venture (“ATC Europe”) comprised primarily of the Company’s operations in Germany and France, (iii) an approximate 75% controlling interest, and the South African investors hold an approximate 25% noncontrolling interest, in a subsidiary of the Company in South Africa and (iv) a 51% controlling interest in ATC Telecom Infrastructure Private Limited (“ATC TIPL”), formerly Viom Networks Limited (“Viom”), in India. Significant Accounting Policies and Use of Estimates —The preparation of financial statements in conformity with accounting principles generally accepted in the United States (“GAAP”) requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Actual results may differ from those estimates, and such differences could be material to the accompanying consolidated financial statements. The significant estimates in the accompanying consolidated financial statements include impairment of long-lived assets (including goodwill), asset retirement obligations, revenue recognition, rent expense, income taxes and accounting for business combinations and acquisitions of assets. The Company considers events or transactions that occur after the balance sheet date but before the financial statements are issued as additional evidence for certain estimates or to identify matters that require additional disclosure. Changes to Prior Year Amounts —The Company has converted its disclosure from thousands to millions and, as a result, any necessary rounding adjustments have been made to prior year disclosed amounts. Accounts Receivable and Deferred Rent Asset —The Company derives the largest portion of its revenues, corresponding accounts receivable and the related deferred rent asset from a relatively small number of tenants in the telecommunications industry, and 53% of its current year revenues are derived from four tenants. The Company’s deferred rent asset is associated with non-cancellable tenant leases that contain fixed escalation clauses over the terms of the applicable lease in which revenue is recognized on a straight-line basis over the lease term. The Company mitigates its concentrations of credit risk with respect to notes and trade receivables and the related deferred rent assets by actively monitoring the creditworthiness of its borrowers and tenants. In recognizing tenant revenue, the Company assesses the collectibility of both the amounts billed and the portion recognized in advance of billing on a straight-line basis. This assessment takes tenant credit risk and business and industry conditions into consideration to ultimately determine the collectibility of the amounts billed. To the extent the amounts, based on management’s estimates, may not be collectible, recognition is deferred until such point as collectibility is determined to be reasonably assured. Any amounts that were previously recognized as revenue and subsequently determined to be uncollectible are charged to bad debt expense included in Selling, general, administrative and development expense in the accompanying consolidated statements of operations. Accounts receivable is reported net of allowances for doubtful accounts related to estimated losses resulting from a tenant’s inability to make required payments and allowances for amounts invoiced whose collectibility is not reasonably assured. These allowances are generally estimated based on payment patterns, days past due and collection history, and incorporate changes in economic conditions that may not be reflected in historical trends, such as tenants in bankruptcy, liquidation or reorganization. Receivables are written-off against the allowances when they are determined to be uncollectible. Such determination includes analysis and consideration of the particular conditions of the account. Changes in the allowances were as follows: Year Ended December 31, 2017 2016 2015 Balance as of January 1, $ 45.9 $ 23.1 $ 17.3 Current year increases 87.2 50.0 19.9 Write-offs, recoveries and other (1) (2.1 ) (27.2 ) (14.1 ) Balance as of December 31, $ 131.0 $ 45.9 $ 23.1 _______________ (1) Recoveries includes recognition of revenue resulting from collections of previously reserved amounts. Functional Currency —The functional currency of each of the Company’s foreign operating subsidiaries is the respective local currency, except for Costa Rica, where the functional currency is the U.S. Dollar. All foreign currency assets and liabilities held by the subsidiaries are translated into U.S. Dollars at the exchange rate in effect at the end of the applicable fiscal reporting period and all foreign currency revenues and expenses are translated at the average monthly exchange rates. Translation adjustments are reflected in equity as a component of Accumulated Other Comprehensive Income Loss (“AOCL”) in the consolidated balance sheets and included as a component of Comprehensive income (loss) in the consolidated statements of comprehensive income (loss). Gains and losses on foreign currency transactions are reflected in Other expense in the consolidated statements of operations. However, the effect from fluctuations in foreign currency exchange rates on intercompany debt that for which repayment is not anticipated in the foreseeable future is reflected in AOCL in the consolidated balance sheets and included as a component of comprehensive income (loss). During the year ended December 31, 2017 , the Company recorded net foreign currency losses of $25.1 million , of which $51.6 million was recorded in AOCL and $(26.5) million was recorded in Other expense. Cash and Cash Equivalents —Cash and cash equivalents include cash on hand, demand deposits and short-term investments with original maturities of three months or less. The Company maintains its deposits at high quality financial institutions and monitors the credit ratings of those institutions. Restricted Cash— Restricted cash includes cash pledged as collateral to secure obligations and all cash whose use is otherwise limited by contractual provisions. The reconciliation of cash and cash equivalents, and restricted cash reported within the statement of financial position that sum to the total of the same such amounts shown in the statement of cash flows is as follows: Year Ended December 31, 2017 2016 2015 Cash and cash equivalents $ 802.1 $ 787.2 $ 320.7 Restricted cash 152.8 149.3 142.2 Total cash, cash equivalents and restricted cash $ 954.9 $ 936.5 $ 462.9 Short-Term Investments— Short-term investments consists of highly liquid investments with original maturities in excess of three months. Property and Equipment —Property and equipment is recorded at cost or, in the case of acquired properties at estimated fair value on the date acquired. Cost for self-constructed towers includes direct materials and labor, capitalized interest and certain indirect costs associated with construction of the tower, such as transportation costs, employee benefits and payroll taxes. The Company begins the capitalization of costs during the pre-construction period, which is the period during which costs are incurred to evaluate the site, and continues to capitalize costs until the tower is substantially completed and ready for occupancy by a tenant. Labor and related costs capitalized for the years ended December 31, 2017 , 2016 and 2015 were $50.9 million , $47.7 million and $44.7 million , respectively. Capitalized interest costs were not material for the years ended December 31, 2017 , 2016 and 2015 . Expenditures for repairs and maintenance are expensed as incurred. Augmentation and improvements that extend an asset’s useful life or enhance capacity are capitalized. Depreciation expense is recorded using the straight-line method over the assets’ estimated useful lives. Towers and related assets on leased land are depreciated over the shorter of the estimated useful life of the asset or the term of the corresponding ground lease, taking into consideration lease renewal options and residual value. Towers or assets acquired through capital leases are recorded net at the present value of future minimum lease payments or the fair value of the leased asset at the inception of the lease. Property and equipment and assets held under capital leases are amortized over the shorter of the applicable lease term or the estimated useful life of the respective assets for periods generally not exceeding twenty years. The Company reviews its tower portfolio for indicators of impairment on an individual tower basis. Impairments primarily result from a tower not having current tenant leases or from having expenses in excess of revenues. The Company reviews other long-lived assets for impairment whenever events, changes in circumstances or other evidence indicate that the carrying amount of the Company’s assets may not be recoverable. The Company records impairment charges in Other operating expenses in the consolidated statements of operations in the period in which the Company identifies such impairment. Goodwill and Other Intangible Assets —The Company reviews goodwill for impairment at least annually (as of December 31) or whenever events or circumstances indicate the carrying value of an asset may not be recoverable. Goodwill is recorded in the applicable segment and assessed for impairment at the reporting unit level. The Company utilizes the two-step impairment test and employs a discounted cash flow analysis when testing goodwill for impairment. The key assumptions utilized in the discounted cash flow analysis include current operating performance, terminal sales growth rate, management’s expectations of future operating results and cash requirements, the current weighted average cost of capital and an expected tax rate. Under the first step of the test, the Company compares the fair value of the reporting unit, as calculated under an income approach using future discounted cash flows, to the carrying amount of the applicable reporting unit. If the carrying amount exceeds the fair value, the Company conducts the second step of this test, in which the implied fair value of the applicable reporting unit’s goodwill is compared to the carrying amount of that goodwill. If the carrying amount of goodwill exceeds its implied fair value, an impairment loss would be recognized for the amount of the excess. During the years ended December 31, 2017 , 2016 and 2015 , no potential impairment was identified under the first step of the test, as the fair value of each of the reporting units was in excess of its carrying amount. Intangible assets that are separable from goodwill and are deemed to have a definite life are amortized over their useful lives, generally ranging from three to twenty years and are evaluated separately for impairment at least annually or whenever events or circumstances indicate that the carrying amount of an asset may not be recoverable. The Company reviews its network location intangible assets for indicators of impairment on an individual tower basis. Impairments primarily result from a tower not having current tenant leases or from having expenses in excess of revenues. The Company monitors its tenant-related intangible assets on a tenant by tenant basis for indicators of impairment, such as high levels of turnover or attrition, non-renewal of a significant number of contracts or the cancellation or termination of a relationship. The Company assesses recoverability by determining whether the carrying amount of the related assets will be recovered primarily through projected undiscounted future cash flows. If the Company determines that the carrying amount of an asset may not be recoverable, the Company measures any impairment loss based on the projected future discounted cash flows to be provided from the asset or available market information relative to the asset’s fair value, as compared to the asset’s carrying amount. The Company records impairment charges in Other operating expenses in the consolidated statements of operations in the period in which the Company identifies such impairment. Derivative Financial Instruments —Derivatives are recorded on the consolidated balance sheet at fair value. If a derivative is designated as a cash flow hedge, the effective portions of changes in the fair value of the derivative are recorded in AOCL, as well as a component of comprehensive income (loss), and are recognized in the results of operations when the hedged item affects earnings. Changes in fair value of the ineffective portions of cash flow hedges are recognized in the results of operations. For derivative instruments that are designated and qualify as fair value hedges, changes in value of the derivatives are recorded in Other expense in the consolidated statements of operations in the current period, along with the offsetting gain or loss on the hedged item attributable to the hedged risk. For derivative instruments not designated as hedging instruments, changes in fair value are recognized in the results of operations in the period that the change occurs. The primary risks managed through the use of derivative instruments is interest rate risk, exposure to changes in the fair value of debt attributable to interest rate risk and currency risk. From time to time, the Company enters into interest rate swap agreements or foreign currency contracts to manage exposure to these risks. Under these agreements, the Company is exposed to counterparty credit risk to the extent that a counterparty fails to meet the terms of a contract. The Company’s exposure is limited to the current value of the contract at the time the counterparty fails to perform. The Company assesses, both at the inception of the hedge and on an ongoing basis, whether the derivatives that are used in hedging transactions are highly effective in offsetting changes in cash flows or fair values of hedged items. The Company does not hold derivatives for trading purposes. Fair Value Measurements —The Company determines the fair value of its financial instruments based on the fair value hierarchy, which requires an entity to maximize the use of observable inputs and minimize the use of unobservable inputs when measuring fair value. Asset Retirement Obligations —When required, the Company recognizes the fair value of obligations to remove its tower assets and remediate the leased land upon which certain of its tower assets are located. Generally, the associated retirement costs are capitalized as part of the carrying amount of the related tower assets and depreciated over their estimated useful lives and the liability is accreted through the obligation’s estimated settlement date. Fair value estimates of asset retirement obligations generally involve discounting of estimated future cash flows associated with takedown costs. Periodic accretion of such liabilities due to the passage of time is included in Depreciation, amortization and accretion expense in the consolidated statements of operations. Adjustments are also made to the asset retirement obligation liability to reflect changes in the estimates of timing and amount of expected cash flows, with an offsetting adjustment made to the related long-lived tangible asset. The significant assumptions used in estimating the Company’s aggregate asset retirement obligation are: timing of tower removals; cost of tower removals; timing and number of land lease renewals; expected inflation rates; and credit-adjusted, risk-free interest rates that approximate the Company’s incremental borrowing rate. Income Taxes —As a REIT, the Company generally is not subject to U.S. federal income taxes on income generated by its U.S. REIT operations. However, the Company remains obligated to pay U.S. federal income taxes on certain earnings and continues to be subject to taxation in its foreign jurisdictions. Accordingly, the consolidated financial statements reflect provisions for federal, state, local and foreign income taxes. The Company recognizes deferred tax assets and liabilities for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax basis, as well as operating loss and tax credit carryforwards. The Company measures deferred tax assets and liabilities using enacted tax rates expected to apply to taxable income in the years in which those temporary differences and carryforwards are expected to be recovered or settled. The effect on deferred tax assets and liabilities as a result of a change in tax rates is recognized in income in the period that includes the enactment date. The Company periodically reviews its deferred tax assets, and provides valuation allowances if, based on the available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. Management assesses the available positive and negative evidence to estimate if sufficient future taxable income will be generated to use the existing deferred tax assets. Valuation allowances would be reversed as a reduction to the provision for income taxes if related deferred tax assets are deemed realizable based on changes in facts and circumstances relevant to the assets’ recoverability. The Company classifies uncertain tax positions as non-current income tax liabilities unless expected to be paid within one year. The Company reports penalties and tax-related interest expense as a component of the income tax provision and interest income from tax refunds as a component of Other expense in the consolidated statements of operations. Other Comprehensive Income (Loss) —Other comprehensive income (loss) refers to items excluded from net income that are recorded as an adjustment to equity, net of tax. The Company’s other comprehensive income (loss) primarily consisted of changes in fair value of effective derivative cash flow hedges, foreign currency translation adjustments and reclassification of unrealized losses on effective derivative cash flow hedges. The AOCL balance included foreign currency translation losses of $2.0 billion , $2.0 billion and $1.8 billion for the years ended December 31, 2017 , 2016 and 2015 , respectively. Distributions —As a REIT, the Company must annually distribute to its stockholders an amount equal to at least 90% of its REIT taxable income (determined before the deduction for distributed earnings and excluding any net capital gain). Generally, the Company has distributed, and expects to continue to distribute, all or substantially all of its REIT taxable income after taking into consideration its utilization of net operating losses (“NOLs”). The amount, timing and frequency of future distributions will be at the sole discretion of the Board of Directors and will depend upon various factors, a number of which may be beyond the Company’s control, including the Company’s financial condition and operating cash flows, the amount required to maintain its qualification for taxation as a REIT and reduce any income and excise taxes that the Company otherwise would be required to pay, limitations on distributions in the Company’s existing and future debt and preferred equity instruments, the Company’s ability to utilize NOLs to offset the Company’s distribution requirements, limitations on its ability to fund distributions using cash generated through its TRSs and other factors that the Board of Directors may deem relevant. Acquisitions —For acquisitions that meet the definition of a business combination, the Company applies the acquisition method of accounting where assets acquired and liabilities assumed are recorded at fair value at the date of each acquisition, and the results of operations are included with those of the Company from the dates of the respective acquisitions. Any excess of the purchase price paid by the Company over the amounts recognized for assets acquired and liabilities assumed is recorded as goodwill. The Company continues to evaluate acquisitions for a period not to exceed one year after the applicable acquisition date of each transaction to determine whether any additional adjustments are needed to the allocation of the purchase price paid for the assets acquired and liabilities assumed. All other acquisitions are accounted for as asset acquisitions and the purchase price is allocated to the net assets acquired with no recognition of goodwill. The purchase price is not subsequently adjusted. The fair value of the assets acquired and liabilities assumed is typically determined by using either estimates of replacement costs or discounted cash flow valuation methods. When determining the fair value of tangible assets acquired, the Company must estimate the cost to replace the asset with a new asset taking into consideration such factors as age, condition and the economic useful life of the asset. When determining the fair value of intangible assets acquired and liabilities assumed, the Company must estimate the applicable discount rate and the timing and amount of future tenant cash flows, including rate and terms of renewal and attrition. Revenue Recognition —The Company’s revenue from leasing and similar arrangements, including fixed escalation clauses present in non-cancellable agreements, is reported on a straight-line basis over the term of the respective agreements when collectibility is reasonably assured. Escalation clauses tied to the Consumer Price Index (“CPI”) or other inflation-based indices, and other incentives present in agreements with the Company’s tenants are excluded from the straight-line calculation. Total property straight-line revenues for the years ended December 31, 2017 , 2016 and 2015 were $194.4 million , $131.7 million and $155.0 million , respectively. Amounts billed upfront in connection with the execution of lease and other agreements are initially deferred and reflected in Unearned revenue in the accompanying consolidated balance sheets and recognized as revenue over the terms of the applicable agreements. Amounts billed or received for services prior to being earned are deferred and reflected in Unearned revenue in the accompanying consolidated balance sheets until the criteria for recognition have been met. Services revenues are derived under contracts or arrangements with customers that provide for billings either on a fixed price basis or a variable price basis, which includes factors such as time and expenses. Revenues are recognized as or when services are performed, and may include estimates for percentage completed. Amounts billed or received for services prior to being earned are deferred and reflected in Unearned revenue in the accompanying consolidated balance sheets until the criteria for recognition have been met. Rent Expense —Many of the leases underlying the Company’s tower sites have fixed rent escalations, which provide for periodic increases in the amount of ground rent payable by the Company over time. In addition, certain of the Company’s tenant leases require the Company to exercise available renewal options pursuant to the underlying ground lease if the tenant exercises its renewal option. The Company calculates straight-line ground rent expense for these leases based on the fixed non-cancellable term of the underlying ground lease plus all periods, if any, for which failure to renew the lease imposes an economic penalty to the Company such that renewal appears to be reasonably assured. Total property straight-line ground rent expense for the years ended December 31, 2017 , 2016 and 2015 was $62.3 million , $67.8 million and $56.1 million , respectively. The Company records a liability for straight-line ground rent expense in Other non-current liabilities. The Company records prepaid ground rent in Prepaid and other current assets and Notes receivable and other non-current assets in the accompanying consolidated balance sheets according to the anticipated period of benefit. Selling, General, Administrative and Development Expense —Selling, general and administrative expense consists of overhead expenses related to the Company’s property and services operations and corporate overhead costs not specifically allocable to any of the Company’s individual business operations. Development expense consists of costs related to the Company’s acquisition efforts, costs associated with new business initiatives and project cancellation costs. Stock-Based Compensation —Stock-based compensation expense is measured at the accounting measurement date based on the fair value of the award and is generally recognized as an expense over the service period, which typically represents the vesting period. The Company provides for accelerated vesting and extended exercise periods of stock options and restricted stock units upon an employee’s death or permanent disability, or upon an employee’s qualified retirement, provided certain eligibility criteria are met. Accordingly, the Company recognizes compensation expense for stock options and time-based restricted stock units (“RSUs”) over the shorter of (i) the four -year vesting period or (ii) the period from the date of grant to the date the employee becomes eligible for such retirement benefits, which may occur upon grant. The expense recognized includes the impact of forfeitures as they occur. In March 2015, 2016 and 2017, the Company granted performance-based restricted stock units (“PSUs”) to its executive officers. Threshold, target and maximum parameters were established for the metrics for each year in the three -year performance period for the March 2015 grants, and for a three -year performance period for the March 2016 and 2017 grants. The metrics will be used to calculate the number of shares that will be issuable when the awards vest, which may range from zero to 200% of the target amounts. The Company recognizes compensation expense for PSUs over the three -year vesting period, subject to adjustment based on the date the employee becomes eligible for retirement benefits as well as performance relative to grant parameters. The fair value of stock options is determined using the Black-Scholes option-pricing model and the fair value of RSUs and PSUs is based on the fair value of the Company’s common stock on the date of grant. The Company recognizes all stock-based compensation expense in either Selling, general, administrative and development expense, costs of operations or as part of the costs associated with the construction of the tower assets. In connection with the vesting of RSUs, the Company withholds from issuance a number of shares of common stock to satisfy certain employee tax withholding obligations arising from such vesting. The shares withheld are considered constructively retired. The Company recognizes the fair value of the shares withheld in Additional paid-in capital on the consolidated balance sheets. As of December 31, 2017, the Company has withheld from issuance an aggregate of 1,442,506 shares, including 222,751 shares related to the vesting of RSUs during the year ended December 31, 2017. Litigation Costs —The Company periodically becomes involved in various claims and lawsuits that are incidental to its business. The Company regularly monitors the status of pending legal actions to evaluate both the magnitude and likelihood of any potential loss. The Company accrues for these potential losses when it is probable that a liability has been incurred and the amount of loss, or possible range of loss, can be reasonably estimated. Should the ultimate losses on contingencies or litigation vary from estimates, adjustments to those liabilities may be required. The Company also incurs legal costs in connection with these matters and records estimates of these expenses, which are reflected in Selling, general, administrative and development expense in the accompanying consolidated statements of operations. Earnings Per Common Share — Basic and Diluted —Basic net income per common share represents net income attributable to American Tower Corporation common stockholders divided by the weighted average number of common shares outstanding during the period. Diluted net income per common share represents net income attributable to American Tower Corporation common stockholders

PREPAID AND OTHER CURRENT ASSET

PREPAID AND OTHER CURRENT ASSETS	12 Months Ended
	Dec. 31, 2017
Prepaid Expense and Other Assets, Current [Abstract]
Prepaid and Other Current Assets	PREPAID AND OTHER CURRENT ASSETS Prepaid and other current assets consisted of the following as of December 31,: 2017 2016 Prepaid operating ground leases $ 148.6 134.2 Prepaid income tax 136.5 127.1 Unbilled receivables 107.9 57.7 Value added tax and other consumption tax receivables 64.2 31.6 Prepaid assets 39.6 36.3 Other miscellaneous current assets 71.8 54.1 Prepaids and other current assets $ 568.6 $ 441.0

PROPERTY AND EQUIPMENT

PROPERTY AND EQUIPMENT	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Property and Equipment	PROPERTY AND EQUIPMENT Property and equipment (including assets held under capital leases) consisted of the following as of December 31,: Estimated Useful Lives (years) (1) 2017 2016 Towers Up to 20 $ 12,500.5 $ 11,740.5 Equipment 2 - 15 1,423.0 1,176.3 Buildings and improvements 3 - 32 631.4 621.9 Land and improvements (2) Up to 20 2,112.9 1,909.7 Construction-in-progress 282.1 203.4 Total 16,949.9 15,651.8 Less accumulated depreciation (5,848.9 ) (5,134.5 ) Property and equipment, net $ 11,101.0 $ 10,517.3 _______________ (1) Assets on leased land are depreciated over the shorter of the estimated useful life of the asset or the term of the corresponding ground lease taking into consideration lease renewal options and residual value. (2) Estimated useful lives apply to improvements only. Depreciation expense for the years ended December 31, 2017 , 2016 and 2015 was $835.5 million , $758.9 million and $661.4 million , respectively. As of December 31, 2017 , property and equipment included $4,944.2 million and $1,370.4 million of capital lease assets and accumulated depreciation, respectively. As of December 31, 2016 , property and equipment included $4,735.3 million and $1,198.0 million of capital lease assets and accumulated depreciation, respectively. As of December 31, 2017 and 2016, capital lease assets were primarily classified as towers and land and improvements.

GOODWILL AND OTHER INTANGIBLE A

GOODWILL AND OTHER INTANGIBLE ASSETS	12 Months Ended
	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Other Intangible Assets	GOODWILL AND OTHER INTANGIBLE ASSETS The changes in the carrying value of goodwill for the Company’s business segments were as follows: Property Services Total U.S. Asia EMEA Latin America Balance as of January 1, 2016 $ 3,379.2 $ 170.7 $ 132.6 $ 407.4 $ 2.0 $ 4,091.9 Additions — 881.8 (1) 40.4 53.5 — 975.7 Effect of foreign currency translation — (23.2 ) (22.5 ) 48.8 — 3.1 Balance as of January 1, 2017 $ 3,379.2 $ 1,029.3 $ 150.5 $ 509.7 $ 2.0 $ 5,070.7 Additions (2) — 0.4 220.9 264.8 — 486.1 Effect of foreign currency translation — 65.3 33.5 (17.2 ) — 81.6 Balance as of December 31, 2017 $ 3,379.2 $ 1,095.0 $ 404.9 $ 757.3 $ 2.0 $ 5,638.4 _______________ (1) Assumed in the acquisition of Viom (see note 6). (2) Additions consist of $485.1 million resulting from 2017 acquisitions and $1.0 million from revisions to prior year acquisitions resulting from measurement period adjustments. The Company’s other intangible assets subject to amortization consisted of the following: As of December 31, 2017 As of December 31, 2016 Estimated Useful Lives Gross Carrying Value Accumulated Amortization Net Book Value Gross Carrying Value Accumulated Amortization Net Book Value (years) Acquired network location intangibles (1) Up to 20 $ 4,858.8 $ (1,525.3 ) $ 3,333.5 $ 4,622.3 $ (1,280.3 ) $ 3,342.0 Acquired tenant-related intangibles 15-20 11,150.9 (2,754.7 ) 8,396.2 10,130.5 (2,224.1 ) 7,906.4 Acquired licenses and other intangibles 3-20 58.8 (8.1 ) 50.7 28.1 (4.8 ) 23.3 Economic Rights, TV Azteca 70 14.5 (11.6 ) 2.9 13.9 (11.0 ) 2.9 Total other intangible assets $ 16,083.0 $ (4,299.7 ) $ 11,783.3 $ 14,794.8 $ (3,520.2 ) $ 11,274.6 _______________ (1) Acquired network location intangibles are amortized over the shorter of the term of the corresponding ground lease taking into consideration lease renewal options and residual value or up to 20 years, as the Company considers these intangibles to be directly related to the tower assets. The acquired network location intangibles represent the value to the Company of the incremental revenue growth that could potentially be obtained from leasing the excess capacity on acquired communications sites. The acquired tenant-related intangibles typically represent the value to the Company of tenant contracts and relationships in place at the time of an acquisition or similar transaction, including assumptions regarding estimated renewals. The Company amortizes its acquired network location intangibles and tenant-related intangibles on a straight-line basis over the estimated useful lives. As of December 31, 2017 , the remaining weighted average amortization period of the Company’s intangible assets, excluding the TV Azteca Economic Rights detailed in note 5, was 15 years. Amortization of intangible assets for the years ended December 31, 2017 , 2016 and 2015 was $785.9 million , $699.8 million and $568.3 million , respectively. Based on current exchange rates, the Company expects to record amortization expense as follows over the next five years: Year Ending December 31, 2018 $ 810.5 2019 806.7 2020 787.2 2021 768.7 2022 766.1

NOTES RECEIVABLE AND OTHER NON-

NOTES RECEIVABLE AND OTHER NON-CURRENT ASSETS	12 Months Ended
	Dec. 31, 2017
Notes Receivable And Other Long Term Assets [Abstract]
Notes Receivable and Other Non-Current Assets	NOTES RECEIVABLE AND OTHER NON-CURRENT ASSETS Notes receivable and other non-current assets consisted of the following as of December 31,: 2017 2016 Long-term prepaid ground rent $ 552.8 $ 467.8 Notes receivable 83.7 83.7 Other miscellaneous assets 313.6 290.0 Notes receivable and other non-current assets $ 950.1 $ 841.5 TV Azteca Note Receivable —In 2000, the Company loaned TV Azteca, S.A. de C.V. (“TV Azteca”), the owner of a major national television network in Mexico, $119.8 million . The loan has an interest rate of 13.11% , payable quarterly, which at the time of issuance was determined to be below market and therefore a corresponding discount was recorded. The term of the loan is 70 years ; however, the loan may be prepaid by TV Azteca without penalty during the last 50 years of the agreement. The discount on the loan is being amortized to Interest income, TV Azteca, net of interest expense on the Company’s consolidated statements of operations, using the effective interest method over the 70 -year term of the loan. Since inception, TV Azteca has repaid $28.0 million of principal on the loan. As of December 31, 2017 and 2016 , the outstanding balance on the loan was $91.8 million , or $82.9 million , net of discount. TV Azteca Economic Rights —Simultaneous with the signing of the loan agreement, the Company also entered into a 70 -year Economic Rights Agreement with TV Azteca regarding space not used by TV Azteca on approximately 190 of its broadcast towers. In exchange for the issuance of the below market interest rate loan and the annual payment of $1.5 million to TV Azteca (under the Economic Rights Agreement), the Company has the right to market and lease the unused tower space on the broadcast towers (the “Economic Rights”). TV Azteca retains title to these towers and is responsible for their operation and maintenance. The Company is entitled to 100% of the revenues generated from leases with tenants on the unused space and is responsible for any incremental operating expenses associated with those tenants. While the term of the Economic Rights Agreement is 70 years , TV Azteca has the right to purchase, at fair market value, the Economic Rights from the Company at any time during the last 50 years of the agreement. Should TV Azteca elect to purchase the Economic Rights, in whole or in part, it would also be obligated to repay a proportional amount of the loan discussed above at the time of such election. The Company’s obligation to pay TV Azteca $1.5 million annually would also be reduced proportionally. The Company accounted for the annual payment of $1.5 million as a capital lease by initially recording an asset and a corresponding liability of $18.6 million . The capital lease asset also included the original discount on the note. The capital lease asset and original discount on the note aggregated $30.2 million at the time of the transaction and represents the cost to acquire the Economic Rights. The Economic Rights asset was recorded as an intangible asset and is being amortized over the 70 -year life of the Economic Rights Agreement.

ACQUISITIONS

ACQUISITIONS	12 Months Ended
	Dec. 31, 2017
Business Combinations [Abstract]
Acquisitions	ACQUISITIONS The Company evaluates each of its acquisitions under the accounting guidance framework to determine whether to treat an acquisition as an asset acquisition or a business combination. For those transactions treated as asset acquisitions, the purchase price is allocated to the assets acquired and liabilities assumed, with no recognition of goodwill. For those transactions treated as business combinations, the estimates of the fair value of the assets or rights acquired and liabilities assumed at the date of the applicable acquisition are subject to adjustment during the measurement period (up to one year from the particular acquisition date). The primary areas of the accounting for the acquisitions that are not yet finalized relate to the fair value of certain tangible and intangible assets acquired and liabilities assumed, which may include contingent consideration, residual goodwill and any related tax impact. The fair value of these net assets acquired are based on management’s estimates and assumptions, as well as other information compiled by management, including valuations that utilize customary valuation procedures and techniques. While the Company believes that such preliminary estimates provide a reasonable basis for estimating the fair value of assets acquired and liabilities assumed, it evaluates any necessary information prior to finalization of the fair value. During the measurement period, the Company will adjust assets or liabilities if new information is obtained about facts and circumstances that existed as of the acquisition date that, if known, would have resulted in the revised estimated values of those assets or liabilities as of that date. Impact of current year acquisitions —The Company typically acquires communications sites from wireless carriers or other tower operators and subsequently integrates those sites into its existing portfolio of communications sites. The financial results of the Company’s acquisitions have been included in the Company’s consolidated statements of operations for the year ended December 31, 2017 from the date of the respective acquisition. The date of acquisition, and by extension the point at which the Company begins to recognize the results of an acquisition, may depend on, among other things, the receipt of contractual consents, the commencement and extent of leasing arrangements and the timing of the transfer of title or rights to the assets, which may be accomplished in phases. Sites acquired from communications service providers may never have been operated as a business and may instead have been utilized solely by the seller as a component of its network infrastructure. An acquisition may or may not involve the transfer of business operations or employees. For those acquisitions accounted for as business combinations, the Company recognizes acquisition and merger related expenses in the period in which they are incurred and services are received; for transactions accounted for as asset acquisitions, these costs are capitalized as part of the purchase price. Acquisition and merger related costs may include finder’s fees, advisory, legal, accounting, valuation and other professional or consulting fees and general administrative costs directly related to the transaction. Integration costs include incremental and non-recurring costs necessary to convert data, retain employees and otherwise enable the Company to operate new businesses or assets efficiently. The Company records acquisition and merger related expenses for business combinations, as well as integration costs for all acquisitions, in Other operating expenses in the consolidated statements of operations. During the years ended December 31, 2017 , 2016 and 2015 , the Company recorded the following acquisition and merger related expenses for business combinations and integration costs: Year Ended December 31, 2017 2016 2015 Acquisition and merger related expenses $ 16.3 $ 15.9 $ 18.8 Integration costs $ 11.5 $ 9.9 $ 18.1 The Company also recorded aggregate purchase price refunds of $22.2 million during the year ended December 31, 2017. The refunds primarily related to an acquisition in Brazil in 2014 for which the measurement period has closed. 2017 Transactions The estimated aggregate impact of the 2017 acquisitions on the Company’s revenues and gross margin for the year ended December 31, 2017 was approximately $82.1 million and $59.3 million , respectively. The revenues and gross margin amounts also reflect incremental revenues from the addition of new tenants to such sites subsequent to the transaction date. FPS Towers France— On February 15, 2017, ATC Europe acquired 100% of the outstanding shares of FPS Towers (“FPS”) from Antin Infrastructure Partners and the individuals party to the purchase agreement (the “FPS Acquisition”), for total consideration of 727.2 million Euros ( $771.2 million at the date of acquisition). FPS owns and operates nearly 2,500 wireless tower sites in France. The Company made a loan to fund 225.0 million Euros ( $238.6 million at the date of acquisition) of the total consideration. The remainder of the purchase price of 502.2 million Euros ( $532.6 million at the date of acquisition) was funded by the Company and PGGM in proportion to their respective interests in ATC Europe. The Company funded its portion of the purchase price with borrowings under its multicurrency senior unsecured revolving credit facility entered into in June 2013, as amended (the “2013 Credit Facility”) and cash on hand. The acquisition is consistent with the Company’s strategy to expand in selected geographic areas. The acquisition was accounted for as a business combination and was subject to post-closing adjustments. All measurement-period adjustments were finalized as of December 31, 2017. Mexico Acquisition— On November 17, 2017, the Company acquired 100% of the outstanding shares of entities holding urban telecommunications assets in Mexico, including more than 50,000 concrete poles and approximately 2,100 route miles of fiber, for total consideration of $505.8 million , including value-added tax (at the date of acquisition). The acquisition was accounted for as a business combination and is subject to post-closing adjustments. Other Acquisitions— During the year ended December 31, 2017, the Company acquired a total of 2,453 communications sites in the United States, Brazil, Chile, Colombia, Germany, Mexico, Nigeria, Paraguay and Peru for an aggregate purchase price of $814.0 million . Of the aggregate purchase price, $22.5 million is reflected in Accounts payable in the consolidated balance sheet as of December 31, 2017. These acquisitions were accounted for as asset acquisitions. The following table summarizes the allocations of the purchase prices for the fiscal year 2017 acquisitions based upon their estimated fair value at the date of acquisition: EMEA Latin America FPS Towers France (1) Mexico (1) Other (2) (3) Final Allocation Preliminary Allocation Current assets $ 34.5 $ 44.4 $ 12.7 Non-current assets 15.0 — 19.7 Property and equipment 122.9 94.0 290.0 Intangible assets (4): Tenant-related intangible assets 440.7 153.3 364.7 Network location intangible assets 113.0 — 154.3 Other intangible assets 8.5 22.0 — Current liabilities (29.0 ) (28.8 ) (10.5 ) Deferred tax liability (135.4 ) (38.8 ) (2.7 ) Other non-current liabilities (19.9 ) (4.5 ) (14.2 ) Net assets acquired 550.3 241.6 814.0 Goodwill (5) 220.9 264.2 — Fair value of net assets acquired 771.2 505.8 814.0 Debt assumed — — — Purchase price $ 771.2 $ 505.8 $ 814.0 _______________ (1) Accounted for as a business combination. (2) Accounted for as asset acquisitions. (3) Includes 127 sites in Peru held pursuant to long-term capital leases. (4) Tenant-related intangible assets, network location intangible assets and other intangible assets are amortized on a straight-line basis over periods of up to 20 years. (5) Primarily results from purchase accounting adjustments, which are not deductible for tax purposes. 2016 Transactions During the year ended December 31, 2017, post-closing adjustments impacted the 2016 acquisitions as follows: Viom Acquisition— On April 21, 2016, the Company acquired a 51% controlling ownership interest in Viom, a telecommunications infrastructure company that owns and operates wireless communications towers and indoor DAS networks in India (the “Viom Acquisition”). Consideration for the acquisition included 76.4 billion Indian Rupees (“INR”) in cash ( $1.1 billion at the date of acquisition), as well as the assumption of approximately 52.3 billion INR ( $0.8 billion at the date of the acquisition) of existing debt, which included 1.7 billion INR ( $25.1 million at the date of the acquisition) of mandatorily redeemable preference shares issued by Viom (the “Viom Preference Shares”). Other Acquisitions— During the year ended December 31, 2016, the Company acquired a total of 891 communications sites in the United States, Brazil, Chile, Germany, Mexico, Nigeria and South Africa, and a company holding urban telecommunications assets and fiber in Argentina, for an aggregate purchase price of $304.4 million (including contingent consideration of $8.8 million ). The following table summarizes the preliminary and updated allocations of the purchase prices paid and the amounts of assets acquired and liabilities assumed for the fiscal year 2016 acquisitions based upon their estimated fair value at the date of acquisition. Balances are reflected in the accompanying consolidated balance sheet as of December 31, 2017. Preliminary Allocation (1) Updated Allocation Asia Other (2) Asia Other (2) Viom Viom (3) Current assets $ 276.6 $ 25.5 $ 281.9 $ 24.5 Non-current assets 57.6 2.3 52.3 2.3 Property and equipment 702.0 81.5 705.8 81.5 Intangible assets (4): Tenant-related intangible assets 1,369.6 105.6 1,369.6 105.6 Network location intangible assets 666.4 83.6 666.4 83.6 Current liabilities (195.9 ) (14.8 ) (201.1 ) (14.8 ) Deferred tax liability (619.1 ) (43.8 ) (619.1 ) (43.4 ) Other non-current liabilities (102.8 ) (29.4 ) (101.8 ) (29.4 ) Net assets acquired 2,154.4 210.5 2,154.0 209.9 Goodwill (5) 881.8 93.9 882.2 94.5 Fair value of net assets acquired 3,036.2 304.4 3,036.2 304.4 Debt assumed (786.8 ) — (786.8 ) — Redeemable noncontrolling interests (1,100.9 ) — (1,100.9 ) — Purchase Price $ 1,148.5 $ 304.4 $ 1,148.5 $ 304.4 _______________ (1) As reported for the year ended December 31, 2016. (2) Of the total purchase price, $12.1 million was reflected in Accounts payable in the consolidated balance sheet as of December 31, 2016. (3) The allocation of the purchase price for the Viom Acquisition was finalized during the year ended December 31, 2017. (4) Tenant-related intangible assets and network location intangible assets are amortized on a straight-line basis over periods of up to 20 years. (5) Primarily results from purchase accounting adjustments, which are at least partially deductible for tax purposes. Pro Forma Consolidated Results (Unaudited) The following table presents the unaudited pro forma financial results as if the 2017 acquisitions had occurred on January 1, 2016 and the 2016 acquisitions had occurred on January 1, 2015 . The pro forma results do not include any anticipated cost synergies, costs or other integration impacts. Accordingly, such pro forma amounts are not necessarily indicative of the results that actually would have occurred had the transactions been completed on the dates indicated, nor are they indicative of the future operating results of the Company. Year Ended December 31, 2017 2016 Pro forma revenues $ 6,775.3 $ 6,240.6 Pro forma net income attributable to American Tower Corporation common stockholders $ 1,145.5 $ 822.8 Pro forma net income per common share amounts: Basic net income attributable to American Tower Corporation common stockholders $ 2.68 $ 1.94 Diluted net income attributable to American Tower Corporation common stockholders $ 2.65 $ 1.92 Other Signed Acquisitions Idea Cellular Limited —On November 13, 2017, the Company entered into an agreement with Idea Cellular Limited (“Idea”) and Idea’s subsidiary, Idea Cellular Infrastructure Services Limited (“ICISL”), to acquire 100% of the outstanding shares of ICISL, a telecommunications company that owns and operates approximately 9,900 communication sites in India, for cash consideration of approximately 40 billion INR ( $611.4 million at the date of signing), subject to certain adjustments (the “Idea Transaction”). Vodafone India Limited —On November 13, 2017, the Company entered into an agreement with Vodafone India Limited and Vodafone Mobile Services Limited (together, “Vodafone”) to acquire their telecommunications site businesses, which consist of an aggregate of approximately 10,235 communication sites, for aggregate cash consideration of approximately 38.5 billion INR ( $588.4 million at the date of signing), subject to certain adjustments (the “Vodafone Transaction” and, together with the Idea Transaction, the “India Transactions”). Consummation of the India Transactions is subject to certain conditions, including regulatory approval. The India Transactions are expected to close in the first half of 2018. Airtel Tanzania— On March 17, 2016, the Company entered into a definitive agreement with Bharti Airtel Limited, through its subsidiary company Airtel Tanzania Limited (“Airtel Tanzania”), pursuant to which the Company could, subject to a number of conditions, acquire certain of Airtel Tanzania’s communications sites in Tanzania. In light of subsequent legislation in Tanzania, the Company did not extend the agreement beyond the expiration date therein. Accordingly, on March 17, 2017, the agreement expired pursuant to its terms and is no longer in effect. Acquisition-Related Contingent Consideration The Company may be required to pay additional consideration under certain agreements for the acquisition of communications sites if specific conditions are met or events occur. In Ghana, the Company may be required to pay additional consideration upon the conversion of certain barter agreements with other wireless carriers to cash-paying lease agreements. In the United States and South Africa, the Company may be required to pay additional consideration if certain pre-designated tenant leases commence during a specified period of time. A summary of the value of the Company’s contingent consideration obligations are as follows: Year Ended December 31, 2017 Maximum potential value (1) Estimated value at December 31, 2017 Additions Settlements Change in Fair Value Colombia $ — $ — $ — $ — $ (5.4 ) Ghana 0.6 0.6 — — 0.0 South Africa 9.1 9.1 — — (0.9 ) United States 0.4 0.4 — — 0.0 Total $ 10.1 $ 10.1 $ — $ — $ (6.3 ) _______________ (1) The maximum potential value is based on exchange rates at December 31, 2017 . The minimum value would be no less than $9.1 million . For more information regarding acquisition-related contingent consideration, see note 11.

ACCRUED EXPENSES

ACCRUED EXPENSES	12 Months Ended
	Dec. 31, 2017
Accrued Liabilities, Current [Abstract]
Accrued Expenses	ACCRUED EXPENSES Accrued expenses consisted of the following as of December 31,: 2017 2016 Accrued property and real estate taxes $ 154.4 $ 138.4 Payroll and related withholdings 82.2 76.1 Amounts payable to tenants 60.8 32.3 Accrued rent 54.0 51.0 Accrued income tax payable 15.3 11.6 Accrued pass-through costs 59.7 68.5 Accrued construction costs 31.9 28.6 Accrued pass-through taxes 25.3 1.0 Other accrued expenses 370.7 213.0 Accrued expenses $ 854.3 $ 620.5

LONG-TERM OBLIGATIONS

LONG-TERM OBLIGATIONS	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Long-Term Obligations	LONG-TERM OBLIGATIONS Outstanding amounts under the Company’s long-term obligations, reflecting discounts, premiums, debt issuance costs and fair value adjustments due to interest rate swaps consisted of the following as of December 31,: 2017 2016 Contractual Interest Rate (1) Maturity Date (1) 2013 Credit Facility (2) $ 2,075.6 $ 540.0 2.649 % June 28, 2021 Term Loan (2) 994.5 993.9 2.790 % January 31, 2023 2014 Credit Facility (2) 495.0 1,385.0 2.820 % January 31, 2023 4.500% senior notes — 998.7 N/A N/A 3.40% senior notes 999.8 999.7 3.400 % February 15, 2019 7.25% senior notes — 297.0 N/A N/A 2.800% senior notes 746.3 744.9 2.800 % June 1, 2020 5.050% senior notes 698.0 697.4 5.050 % September 1, 2020 3.300% senior notes 746.0 744.8 3.300 % February 15, 2021 3.450% senior notes 645.1 643.8 3.450 % September 15, 2021 5.900% senior notes 497.8 497.3 5.900 % November 1, 2021 2.250% senior notes 572.4 572.8 2.250 % January 15, 2022 4.70% senior notes 696.7 696.0 4.700 % March 15, 2022 3.50% senior notes 990.9 989.3 3.500 % January 31, 2023 3.000% senior notes 692.5 — 3.000 % June 15, 2023 5.00% senior notes 1,002.4 1,002.7 5.000 % February 15, 2024 1.375% senior notes 589.1 — 1.375 % April 4, 2025 4.000% senior notes 741.0 740.0 4.000 % June 1, 2025 4.400% senior notes 495.6 495.2 4.400 % February 15, 2026 3.375% senior notes 984.8 983.4 3.375 % October 15, 2026 3.125% senior notes 397.1 396.7 3.125 % January 15, 2027 3.55% senior notes 742.8 — 3.550 % July 15, 2027 3.600% senior notes 691.1 — 3.600 % January 15, 2028 Total American Tower Corporation debt 16,494.5 14,418.6 Series 2013-1A Securities (3) 499.8 498.6 1.551 % March 15, 2018 Series 2013-2A Securities (4) 1,291.8 1,290.3 3.070 % March 15, 2023 Series 2015-1 Notes (5) 348.0 347.1 2.350 % June 15, 2020 Series 2015-2 Notes (6) 520.1 519.4 3.482 % June 16, 2025 2012 GTP Notes — 179.5 N/A N/A Unison Notes — 133.0 N/A N/A India indebtedness (7) 512.6 549.5 7.90% - 9.55% Various India preference shares (8) 26.1 24.5 10.250 % March 2, 2020 Shareholder loans (9) 100.6 151.1 Various Various Other subsidiary debt (1) (10) 246.1 286.0 Various Various Total American Tower subsidiary debt 3,545.1 3,979.0 Other debt, including capital lease obligations 165.5 135.9 Total 20,205.1 18,533.5 Less current portion long-term obligations (774.8 ) (238.8 ) Long-term obligations $ 19,430.3 $ 18,294.7 _______________ (1) Represents the interest rate or maturity date as of December 31, 2017; interest rate does not reflect the impact of interest rate swap agreements. (2) Accrues interest at a variable rate. (3) Maturity date reflects the anticipated repayment date; final legal maturity is March 15, 2043. (4) Maturity date reflects the anticipated repayment date; final legal maturity is March 15, 2048. (5) Maturity date reflects the anticipated repayment date; final legal maturity is June 15, 2045. (6) Maturity date reflects the anticipated repayment date; final legal maturity is June 15, 2050. (7) Denominated in INR. Includes India working capital facility, remaining debt assumed by the Company in connection with the Viom Acquisition and debt that has been entered into by ATC TIPL. (8) Mandatorily redeemable preference shares (the “Preference Shares”) classified as debt. The Preference Shares are to be redeemed on March 2, 2020 and have a dividend rate of 10.25% per annum. (9) Reflects balances owed to the Company’s joint venture partners in Ghana and Uganda. The Ghana loan is denominated in Ghanaian Cedi (“GHS”) and the Uganda loan is denominated in Ugandan Shillings (“UGX”). (10) Includes the BR Towers debentures and the Brazil Credit Facility (as defined below), which are denominated in Brazilian Reais (“BRL”) and amortize through October 15, 2023 and January 15, 2022, respectively, the South African Credit Facility (as defined below), which is denominated in South African Rand (“ZAR”) and amortizes through December 17, 2020 and the Colombian Credit Facility (as defined below), which is denominated in Colombian Pesos (“COP”) and amortizes through April 24, 2021. American Tower Corporation Debt Bank Facilities —In December 2017, the Company entered into amendment agreements with respect to (i) the 2013 Credit Facility, (ii) its senior unsecured revolving credit facility entered into in January 2012, as amended and restated in September 2014, as further amended (the “2014 Credit Facility”) and (iii) its unsecured term loan entered into in October 2013, as amended (the “Term Loan”), which, among other things, extend the maturity dates by one year to June 28, 2021, January 31, 2023 and January 31, 2023, respectively. In addition, the amendment to the 2013 Credit Facility reduces the Applicable Margins (as defined in the 2013 Credit Facility) and the commitment fees set forth therein. 2013 Credit Facility— The Company has the ability to borrow up to $2.75 billion under the 2013 Credit Facility, which includes a $1.0 billion sublimit for multicurrency borrowings, a $200.0 million sublimit for letters of credit and a $50.0 million sublimit for swingline loans. During the year ended December 31, 2017 , the Company borrowed an aggregate of $3.8 billion and repaid an aggregate of $2.3 billion of revolving indebtedness under the 2013 Credit Facility. The Company primarily used the borrowings to fund acquisitions, repay existing indebtedness and for general corporate purposes. 2014 Credit Facility— The Company has the ability to borrow up to $2.0 billion under the 2014 Credit Facility, which includes a $200.0 million sublimit for letters of credit and a $50.0 million sublimit for swingline loans. During the year ended December 31, 2017 , the Company borrowed an aggregate of $815.0 million and repaid an aggregate of $1.7 billion of revolving indebtedness under the 2014 Credit Facility. The Company primarily used the borrowings to fund acquisitions and for general corporate purposes. The Term Loan, the 2013 Credit Facility and the 2014 Credit Facility do not require amortization of principal and may be paid prior to maturity in whole or in part at the Company’s option without penalty or premium. The Company has the option of choosing either a defined base rate or the London Interbank Offered Rate (“LIBOR”) as the applicable base rate for borrowings under the Term Loan, the 2013 Credit Facility and the 2014 Credit Facility. The interest rate on the 2013 Credit Facility ranges between 0.875% to 1.750% above LIBOR for LIBOR based borrowings or up to 0.750% above the defined base rate for base rate borrowings, in each case based upon the Company’s debt ratings. The interest rates on the Term Loan and the 2014 Credit Facility range between 1.000% to 2.000% above LIBOR for LIBOR based borrowings or up to 1.000% above the defined base rate for base rate borrowings, in each case based upon the Company’s debt ratings. As of December 31, 2017 , the key terms under the 2013 Credit Facility, the 2014 Credit Facility and the Term Loan were as follows: Outstanding Principal Balance Undrawn letters of credit Maturity Date Current margin over LIBOR Current commitment fee (1) 2013 Credit Facility $ 2,075.6 (2) $ 4.0 June 28, 2021 (3) 1.125 % 0.125 % 2014 Credit Facility $ 495.0 (2) $ 6.3 January 31, 2023 (3) 1.250 % 0.150 % Term Loan $ 1,000.0 (2) $ — January 31, 2023 1.250 % N/A _______________ (1) Fee on undrawn portion of each credit facility. (2) Borrowed at LIBOR. (3) Subject to two optional renewal periods. Senior Notes 1.375% Senior Notes Offering— On April 6, 2017, the Company completed a registered public offering of 500.0 million Euros ( $532.2 million at the date of issuance) aggregate principal amount of 1.375% senior unsecured notes due 2025 (the “ 1.375% Notes”). The net proceeds from this offering were approximately 489.8 million Euros (approximately $521.4 million at the date of issuance), after deducting commissions and estimated expenses. The Company used the net proceeds to repay existing indebtedness under the 2013 Credit Facility and for general corporate purposes. The 1.375% Notes will mature on April 4, 2025 and bear interest at a rate of 1.375% per annum. Accrued and unpaid interest on the 1.375% Notes will be payable in Euros in arrears on April 4 of each year, beginning on April 4, 2018. Interest on the 1.375% Notes will be computed on the basis of the actual number of days in the period for which interest is being calculated and the actual number of days from and including the last date on which interest was paid on the 1.375% Notes and commenced accruing on April 6, 2017. 3.55% Senior Notes Offering— On June 30, 2017, the Company completed a registered public offering of $750.0 million aggregate principal amount of 3.55% senior unsecured notes due 2027 (the “ 3.55% Notes”). The net proceeds from this offering were approximately $741.8 million , after deducting commissions and estimated expenses. The Company used the net proceeds to repay existing indebtedness under the 2013 Credit Facility. The 3.55% Notes will mature on July 15, 2027 and bear interest at a rate of 3.55% per annum. Accrued and unpaid interest on the 3.55% Notes will be payable in U.S. Dollars semi-annually in arrears on January 15 and July 15 of each year, beginning on January 15, 2018. Interest on the 3.55% Notes is computed on the basis of a 360-day year comprised of twelve 30-day months and commenced accruing on June 30, 2017. 3.000% Senior Notes and 3.600% Senior Notes Offerings— On December 8, 2017, the Company completed registered public offerings of $700.0 million aggregate principal amount of 3.000% senior unsecured notes due 2023 (the “ 3.000% Notes”) and $700.0 million aggregate principal amount of 3.600% senior unsecured notes due 2028 (the “ 3.600% Notes”). The net proceeds from these offerings were approximately $1,382.9 million , after deducting commissions and estimated expenses. The Company used the net proceeds to repay existing indebtedness under the 2013 Credit Facility and the 2014 Credit Facility. The 3.000% Notes will mature on June 15, 2023 and bear interest at a rate of 3.000% per annum. The 3.600% Notes will mature on January 15, 2028 and bear interest at a rate of 3.600% per annum. Accrued and unpaid interest on the 3.000% Notes will be payable in U.S. Dollars semi-annually in arrears on June 15 and December 15 of each year, beginning on June 15, 2018. Accrued and unpaid interest on the 3.600% Notes will be payable in U.S. Dollars semi-annually in arrears on January 15 and July 15 of each year, beginning on July 15, 2018. Interest on the 3.000% Notes and the 3.600% Notes is computed on the basis of a 360-day year comprised of twelve 30-day months and commenced accruing on December 8, 2017. The Company entered into interest rate swaps, which were designated as fair value hedges at inception, to hedge against changes in fair value of $500.0 million of the $700.0 million under the 3.000% Notes resulting from changes in interest rates. As of December 31, 2017, the interest rate on the 3.000% Notes, after giving effect to the interest rate swap agreements, was 2.49% . The following table outlines key terms related to the Company ’ s outstanding senior notes as of December 31, 2017 : Adjustments to Principal Amount (1) Aggregate Principal Amount 2017 2016 Interest payments due (2) Issue Date Par Call Date (3) 3.40% Notes (4) 1,000.0 (0.2 ) (0.3 ) February 15 and August 15 August 19, 2013 N/A 2.800% Notes 750.0 (3.7 ) (5.1 ) June 1 and December 1 May 7, 2015 May 1, 2020 5.050% Notes 700.0 (2.0 ) (2.6 ) March 1 and September 1 August 16, 2010 N/A 3.300% Notes 750.0 (4.0 ) (5.2 ) February 15 and August 15 January 12, 2016 January 15, 2021 3.450% Notes 650.0 (4.9 ) (6.2 ) March 15 and September 15 August 7, 2014 N/A 5.900% Notes 500.0 (2.2 ) (2.7 ) May 1 and November 1 October 6, 2011 N/A 2.250% Notes (5) 600.0 (27.6 ) (27.2 ) January 15 and July 15 September 30, 2016 N/A 4.70% Notes 700.0 (3.3 ) (4.0 ) March 15 and September 15 March 12, 2012 N/A 3.50% Notes 1,000.0 (9.1 ) (10.7 ) January 31 and July 31 January 8, 2013 N/A 3.000% Notes (6) 700.0 (7.5 ) — June 15 and December 15 December 8, 2017 N/A 5.00% Notes (4) 1,000.0 2.4 2.7 February 15 and August 15 August 19, 2013 N/A 1.375% Notes (7) 600.2 (11.1 ) — April 4 April 6, 2017 January 4, 2025 4.000% Notes 750.0 (9.0 ) (10.0 ) June 1 and December 1 May 7, 2015 March 1, 2025 4.400% Notes 500.0 (4.4 ) (4.8 ) February 15 and August 15 January 12, 2016 November 15, 2025 3.375% Notes 1,000.0 (15.2 ) (16.6 ) April 15 and October 15 May 13, 2016 July 15, 2026 3.125% Notes 400.0 (2.9 ) (3.3 ) January 15 and July 15 September 30, 2016 October 15, 2026 3.55% Notes 750.0 (7.2 ) — January 15 and July 15 June 30, 2017 April 15, 2027 3.600% Notes 700.0 (8.9 ) — January 15 and July 15 December 8, 2017 October 15, 2027 _______________ (1) Includes unamortized discounts, premiums and debt issuance costs and fair value adjustments due to interest rate swaps. (2) Interest payments are due semi-annually for each series of senior notes, except for the 1.375% Notes, for which interest payments are due annually on April 4. (3) The Company will not be required to pay a make-whole premium if redeemed on or after the par call date. (4) The original issue date for the 3.40% Notes and the 5.00% Notes was August 19, 2013. The issue date for the reopened 3.40% Notes and the reopened 5.00% Notes was January 10, 2014. (5) Includes $23.7 million and $22.3 million fair value adjustment due to interest rate swaps in 2017 and 2016, respectively. (6) Includes $0.8 million fair value adjustment due to interest rate swaps. (7) Note is denominated in Euro. The Company may redeem each series of senior notes at any time, subject to the terms of the applicable supplemental indenture, in whole or in part, at a redemption price equal to 100% of the principal amount of the notes plus a make-whole premium, together with accrued interest to the redemption date. In addition, if the Company undergoes a change of control and corresponding ratings decline, each as defined in the applicable supplemental indenture, it may be required to repurchase all of the applicable notes at a purchase price equal to 101% of the principal amount of such notes, plus accrued and unpaid interest (including additional interest, if any), up to but not including the repurchase date. The notes rank equally with all of the Company’s other senior unsecured debt and are structurally subordinated to all existing and future indebtedness and other obligations of its subsidiaries. Each applicable supplemental indenture for the notes contains certain covenants that restrict the Company’s ability to merge, consolidate or sell assets and its (together with its subsidiaries’) ability to incur liens. These covenants are subject to a number of exceptions, including that the Company and its subsidiaries may incur certain liens on assets, mortgages or other liens securing indebtedness if the aggregate amount of such liens does not exceed 3.5 x Adjusted EBITDA, as defined in the applicable supplemental indenture. Redemption of Senior Notes— On February 10, 2017, the Company redeemed all of the outstanding 7.25% senior unsecured notes due 2019 (the “ 7.25% Notes”) at a price equal to 112.0854% of the principal amount, plus accrued and unpaid interest up to, but excluding, February 10, 2017, for an aggregate redemption price of $341.4 million , including $5.1 million in accrued and unpaid interest. The Company recorded a loss on retirement of long-term obligations of $39.2 million , which includes prepayment consideration of $36.3 million and the remaining portion of the unamortized discount and deferred financing costs. Upon completion of the redemption, none of the 7.25% Notes remained outstanding. On July 31, 2017, the Company redeemed all of the outstanding 4.500% senior unsecured notes due 2018 (the “ 4.500% Notes”) at a price equal to 101.3510% of the principal amount, plus accrued and unpaid interest up to, but excluding, July 31, 2017, for an aggregate redemption price of $1.0 billion , including $2.0 million in accrued and unpaid interest. The Company recorded a loss on retirement of long-term obligations of $14.1 million which includes prepayment consideration of $13.5 million and the remaining portion of the unamortized discount and deferred financing costs. Upon completion of the redemption, none of the 4.500% Notes remained outstanding. The redemptions described above were funded with borrowings under the 2013 Credit Facility and cash on hand. American Tower Subsidiary Debt Subsidiary Debt The Company has several securitizations in place. Cash flows generated by the sites that secure the securitized debt are only available for payment of such debt and are not available to pay the Company’s other obligations or the claims of its creditors. However, subject to certain restrictions, the Company holds the right to the excess cash flows not needed to pay the securitized debt and other obligations arising out of the securitizations. The securitized debt is the obligation of the issuers thereof or borrowers thereunder, as applicable, and their subsidiaries, and not of the Company or its other subsidiaries. Secured Tower Revenue Securities, Series 2013-1A and Series 2013-2A —In March 2013, the Company completed a private issuance (the “2013 Securitization”) of $1.8 billion of Secured Tower Revenue Securities, Series 2013-1A (the “Series 2013-1A Securities”) and Series 2013-2A (the “Series 2013-2A Securities,” and together with the Series 2013-1A Securities, the “2013 Securities”) issued by American Tower Trust I (the “Trust”), a trust established by American Tower Depositor Sub, LLC, a wholly owned special purpose subsidiary of the Company. The net proceeds of the transaction were $1.78 billion . The assets of the Trust consist of a nonrecourse loan (the “Loan”) to American Tower Asset Sub, LLC and American Tower Asset Sub II, LLC (the “AMT Asset Subs”), pursuant to a First Amended and Restated Loan and Security Agreement dated as of March 15, 2013 (the “Loan Agreement”). The Loan is secured by (i) mortgages, deeds of trust and deeds to secure debt on substantially all of the 5,178 wireless and broadcast communications towers owned by the AMT Asset Subs (the “2013 Secured Towers”), (ii) a pledge of the AMT Asset Subs’ operating cash flows from the 2013 Secured Towers, (iii) a security interest in substantially all of the AMT Asset Subs’ personal property and fixtures and (iv) the AMT Asset Subs’ rights under the tenant leases and the management agreement entered into in connection with the 2013 Securitization. American Tower Holding Sub, LLC, whose only material assets are its equity interests in each of the AMT Asset Subs, and American Tower Guarantor Sub, LLC, whose only material asset are its equity interests in American Tower Holding Sub, LLC, each have guaranteed repayment of the Loan and pledged their equity interests in their respective subsidiary or subsidiaries as security for such payment obligations. The 2013 Securities were issued in two separate series of the same class pursuant to a First Amended and Restated Trust and Servicing Agreement, with terms identical to the Loan. The effective weighted average life and interest rate of the 2013 Securities was 8.6 years and 2.648% , respectively, as of the date of issuance. American Tower Secured Revenue Notes, Series 2015-1, Class A and Series 2015-2, Class A —In May 2015, GTP Acquisition Partners I, LLC (“GTP Acquisition Partners”), one of the Company’s wholly owned subsidiaries, refinanced existing debt with cash on hand and proceeds from a private issuance (the “2015 Securitization”) of $350.0 million of American Tower Secured Revenue Notes, Series 2015-1, Class A (the “Series 2015-1 Notes”) and $525.0 million of American Tower Secured Revenue Notes, Series 2015-2, Class A (the “Series 2015-2 Notes,” and together with the Series 2015-1 Notes, the “2015 Notes”). The 2015 Notes are secured by (i) mortgages, deeds of trust and deeds to secure debt on substantially all of the 3,583 communications sites (the “2015 Secured Sites”) owned by GTP Acquisition Partners and its subsidiaries (the “GTP Entities”) and their operating cash flows, (ii) a security interest in substantially all of the personal property and fixtures of the GTP Entities, including GTP Acquisition Partners’ equity interests in its subsidiaries and (iii) the rights of the GTP Entities under a management agreement. American Tower Holding Sub II, LLC, whose only material assets are its equity interests in GTP Acquisition Partners, has guaranteed repayment of the 2015 Notes and pledged its equity interests in GTP Acquisition Partners as security for such payment obligations. The 2015 Notes were issued by GTP Acquisition Partners pursuant to a Third Amended and Restated Indenture and related series supplements, each dated as of May 29, 2015 (collectively, the “2015 Indenture”), between the GTP Entities and The Bank of New York Mellon, as trustee. The effective weighted average life and interest rate of the 2015 Notes was 8.1 years and 3.029% , respectively, as of the date of issuance. Under the terms of the Loan Agreement and 2015 Indenture, amounts due will be paid from the cash flows generated by the 2013 Secured Towers or the 2015 Secured Sites, respectively, which must be deposited into certain reserve accounts, and thereafter distributed solely pursuant to the terms of the Loan Agreement or 2015 Indenture, as applicable. On a monthly basis, after payment of all required amounts under the Loan Agreement or 2015 Indenture, as applicable, including interest payments, subject to the conditions described below, the excess cash flows generated from the operation of such assets are released to the AMT Asset Subs or GTP Acquisition Partners, as applicable, which can then be distributed to, and used by, the Company. In order to distribute any excess cash flow to the Company, the AMT Asset Subs and GTP Acquisition Partners must each maintain a specified debt service coverage ratio (the “DSCR”), which is generally calculated as the ratio of the net cash flow (as defined in the applicable agreement) to the amount of interest, servicing fees and trustee fees required to be paid over the succeeding 12 months on the principal amount of the Loan or the 2015 Notes, as applicable, that will be outstanding on the payment date following such date of determination. If the DSCR were equal to or below 1.30 x (the “Cash Trap DSCR”) for any quarter, then all cash flow in excess of amounts required to make debt service payments, fund required reserves, pay management fees and budgeted operating expenses and make other payments required under the applicable transaction documents, referred to as excess cash flow, will be deposited into a reserve account (the “Cash Trap Reserve Account”) instead of being released to the AMT Asset Subs or GTP Acquisition Partners, as applicable. The funds in the Cash Trap Reserve Account will not be released to the AMT Asset Subs or GTP Acquisition Partners, as applicable, unless the DSCR exceeds the Cash Trap DSCR for two consecutive calendar quarters. Additionally, an “amortization period” commences if, as of the end of any calendar quarter, the DSCR is equal to or below 1.15 x (the “Minimum DSCR”) and will continue to exist until the DSCR exceeds the Minimum DSCR for two consecutive calendar quarters. With respect to the 2013 Securities, an “amortization period” also commences if, on the anticipated repayment date the component of the Loan corresponding to the applicable subclass of the 2013 Securities has not been repaid in full, provided that such amortization period shall apply with respect to such component that has not been repaid in full. If either series of the 2015 Notes have not been repaid in full on the applicable anticipated repayment date, additional interest will accrue on the unpaid principal balance of the applicable series of the 2015 Notes, and such series will begin to amortize on a monthly basis from excess cash flow. During an amortization period, all excess cash flow and any amounts then in the applicable Cash Trap Reserve Account would be applied to pay the principal of the Loan or the 2015 Notes, as applicable, on each monthly payment date. The Loan and the 2015 Notes may be prepaid in whole or in part at any time, provided such payment is accompanied by the applicable prepayment consideration. If the prepayment occurs within 12 months of the anticipated repayment date with respect to the Series 2013-1A Securities or the Series 2015-1 Notes, or 18 months of the anticipated repayment date with respect to the Series 2013-2A Securities or the Series 2015-2 Notes, no prepayment consideration is due. The Loan may be defeased in whole at any time prior to the anticipated repayment date for any component of the Loan then outstanding. The Loan Agreement and the 2015 Indenture include operating covenants and other restrictions customary for transactions subject to rated securitizations. Among other things, the AMT Asset Subs and the GTP Entities, as applicable, are prohibited from incurring other indebtedness for borrowed money or further encumbering their assets subject to customary carve-outs for ordinary course trade payables and permitted encumbrances (as defined in the Loan Agreement or the 2015 Indenture, as applicable). The organizational documents of the AMT Asset Subs and the GTP Entities contain provisions consistent with rating agency securitization criteria for special purpose entities, including the requirement that they maintain independent directors. The Loan Agreement and the 2015 Indenture also contain certain covenants that require the AMT Asset Subs or GTP Acquisition Partners, as applicable, to provide the respective trustee with regular financial reports and operating budgets, promptly notify such trustee of events of default and material breaches under the Loan Agreement and other agreements related to the 2013 Secured Towers or the 2015 Indenture and other agreements related to the 2015 Secured Sites, as applicable, and allow the applicable trustee reasonable access to the sites, including the right to conduct site investigations. A failure to comply with the covenants in the Loan Agreement or the 2015 Indenture could prevent the AMT Asset Subs or GTP Acquisition Partners, as applicable, from distributing excess cash flow to the Company. Furthermore, if the AMT Asset Subs or GTP Acquisition Partners were to default on the Loan or a series of the 2015 Notes, the applicable trustee may seek to foreclose upon or otherwise convert the ownership of all or any portion of the 2013 Secured Towers or the 2015 Secured Sites, respectively, in which case the Company could lose the revenue associated with those assets. With respect to the 2015 Notes, upon the occurrence and during an event of default, the applicable trustee may, in its discretion or at the direction of holders of more than 50% of the aggregate outstanding principal of any series of the 2015 Notes, declare such series of 2015 Notes immediately due and payable, in which case any excess cash flow would need to be used to pay holders of such notes. Further, under the Loan Agreement and the 2015 Indenture, the AMT Asset Subs or GTP Acquisition Partners, respectively, are required to maintain reserve accounts, including for amounts received or due from tenants related to future periods, property taxes, insurance, ground rents, certain expenses and debt service. Based on the terms of the Loan Agreement and the 2015 Indenture, all rental cash receipts received for each month are reserved for the succeeding month and held in an account controlled by the applicable trustee and then released. The $90.4 million held in the reserve accounts with respect to the 2013 Securitization and the $16.9 million held in the reserve accounts with respect to the 2015 Securitization as of December 31, 2017 are classified as Restricted cash on the Company’s accompanying consolidated balance sheets. Repayment of 2012 GTP Notes and Unison Notes —On February 15, 2017, the Company repaid the $173.5 million remaining principal amount outstanding under the Secured Cellular Site Revenue Notes, Series 2012-2 Class A, Series 2012-2 Class B and Series 2012-2 Class C issued by GTP Cellular Sites, LLC, plus prepayment consideration and accrued and unpaid interest. The Company recorded a loss on retirement of long-term obligations of $1.8 million , which includes prepayment consideration of $7.2 million offset by the remaining portion of the unamortized premium. On February 15, 2017, the Company repaid the $129.0 million principal amount outstanding under the Secured Cellular Site Revenue Notes, Series 2010-2, Class C and Series 2010-2, Class F issued by Unison Ground Lease Funding, LLC, plus prepayment consideration and accrued and unpaid interest. The Company recorded a loss on retirement of long-term obligations of $14.5 million , which includes prepayment consideration of $18.3 million offset by the remaining portion of the unamortized premium. The repayments described above were funded with borrowings under the 2013 Credit Facility and cash on hand. India indebtedness— Amounts outstanding and key terms of the India indebtedness consisted of the following as of December 31, 2017 (in millions, except percentages): Amount Outstanding (INR) Amount Outstanding (USD) Interest Rate (Range) Maturity Date (Range) Term loans 26,740 $ 418.7 7.90% - 8.65% January 24, 2018 - November 30, 2024 Debenture 6,000 $ 93.9 9.55 % April 28, 2020 Working capital facilities 0 $ 0 8.05% - 8.75% March 18, 2018 - October 23, 2018 The India indebtedness includes several term loans, ranging from 1 to 10 years, which are generally secured by the borrower’s short-term and long-term assets. Each of the term loans bear interest at the applicable bank’s Marginal Cost of Funds based Lending Rate (as defined in the applicable agreement) or base rate, plus a spread. Interest rates on the term loans are fixed until certain reset dates. Generally, the term loans can be repaid without penalty on the reset dates; repayments at dates other than the reset dates are subject to prepayment penalties, typically of 1% to 2% . Scheduled repayment terms include either ratable or staggered amortization with repayments typically commencing between 6 and 36 months after the initial disbursement of funds. The debenture is secured by the borrower’s long-term assets, including property and equipment and intangible assets. The debenture bears interest at a base rate plus a spread of 0.6% . The base rate is set in advance for each quarterly coupon period. Should the actual base rate be between 9.75% and 10.25% , the revised base rate is assumed to be 10.00% for purposes of the reset. Additionally, the spread is subject to reset 36 and 48 months from the issuance date of April 27, 2015. The holders of the debenture must reach a consensus on the revised spread and the borrower must redeem all of the debentures held by holders from whom consensus is not achieved. Additionally, the debenture is required to be redeemed by the borrower if it does not maintain a minimum credit rating. The India indebtedness includes several working capital facilities, most of which are subject to annual renewal, and which are generally secured by the borrower’s short-term and long-term assets. The working capital facilities bear interest at rates that are comprised of the applicable bank’s Marginal Cost of Funds based Lending Rate (as defined in the applicable agreement) or base rate, plus a spread. Generally, the working capital facilities are payable on demand prior to maturity. Preference shares— On March 2, 2017, ATC TIPL issued 166,666,666 Preference Shares and used the proceeds to redeem the Viom Preference Shares. As of December 31, 2017 , ATC TIPL had 166,666,666 Preference Shares outstanding, which are required to be redeemed in cash. Accordingly, the Company recognized debt of 1.67 billion INR ( $26.1 million ) related to the Preference Shares outstanding on the consolidated balance sheet. Unless redeemed earlier, the Preference Shares will be redeemed on March 2, 2020 in an amount equal to ten INR per share along with a redemption premium, as defined in the investment agreement, which equates to a compounded return of 10.25% per annum. Other Subsidiary Debt— The Company’s other subsidiary debt includes (i) publicly issued simple debentures in Brazil (the “BR Towers Debentures”) issued by a subsidiary of BR Towers and assumed by the Company in its acquisition of BR Towers, (ii) a credit facility entered into by one of the Company’s S

ASSET RETIREMENT OBLIGATIONS

ASSET RETIREMENT OBLIGATIONS	12 Months Ended
	Dec. 31, 2017
Asset Retirement Obligation [Abstract]
Asset Retirement Obligations	ASSET RETIREMENT OBLIGATIONS The changes in the carrying amount of the Company’s asset retirement obligations were as follows: 2017 2016 Beginning balance as of January 1, $ 965.5 $ 856.9 Additions 33.4 64.1 Accretion expense 94.5 67.0 Revisions in estimates (1) 86.6 (21.1 ) Settlements (4.7 ) (1.4 ) Balance as of December 31, $ 1,175.3 $ 965.5 _______________ (1) Revisions in estimates include an increase to the liability of $13.0 million and $9.6 million related to foreign currency translation for the years ended December 31, 2017 and 2016, respectively. As of December 31, 2017 , the estimated undiscounted future cash outlay for asset retirement obligations was $3.0 billion .

FAIR VALUE MEASUREMENTS

FAIR VALUE MEASUREMENTS	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value Measurements	FAIR VALUE MEASUREMENTS The Company determines the fair value of its financial instruments based on the fair value hierarchy, which requires an entity to maximize the use of observable inputs and minimize the use of unobservable inputs when measuring fair value. Below are the three levels of inputs that may be used to measure fair value: Level 1 Quoted prices in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date. Level 2 Observable inputs other than Level 1 prices, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Level 3 Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. Items Measured at Fair Value on a Recurring Basis —The fair value of the Company’s financial assets and liabilities that are required to be measured on a recurring basis at fair value was as follows: December 31, 2017 December 31, 2016 Fair Value Measurements Using Fair Value Measurements Using Level 1 Level 2 Level 3 Level 1 Level 2 Level 3 Assets: Short-term investments (1) $ 1.0 — — $ 4.0 — — Interest rate swap agreements — — — — $ 0.0 — Embedded derivative in lease agreement — — $ 12.4 — — $ 13.3 Liabilities: Interest rate swap agreements — $ 29.0 — — $ 24.7 — Acquisition-related contingent consideration — — $ 10.1 — — $ 15.4 Fair value of debt related to interest rate swap agreements $ (24.5 ) — — $ (22.3 ) — — _______________ (1) Consists of highly liquid investments with original maturities in excess of three months. Interest Rate Swap Agreements The fair value of the Company’s interest rate swap agreements is determined using pricing models with inputs that are observable in the market or can be derived principally from, or corroborated by, observable market data. For derivative instruments that are designated and qualify as fair value hedges, changes in value of the derivatives are recognized in the consolidated statement of operations in the current period, along with the offsetting gain or loss on the hedged item attributable to the hedged risk. For derivative instruments that are designated and qualify as cash flow hedges, the Company records the change in fair value for the effective portion of the cash flow hedges in AOCL in the consolidated balance sheets and reclassifies a portion of the value from AOCL into Interest expense on a quarterly basis as the cash flows from the hedged item affects earnings. The Company records the settlement of interest rate swap agreements in (Loss) gain on retirement of long-term obligations in the consolidated statements of operations in the period in which the settlement occurs. In December 2017, the Company entered into three interest rate swap agreements with an aggregate notional value of $500.0 million related to the 3.000% Notes. These interest rate swaps, which were designated as fair value hedges at inception, were entered into to hedge against changes in fair value of the 3.000% Notes resulting from changes in interest rates. The interest rate swap agreements require the Company to pay interest at a variable interest rate of one-month LIBOR plus applicable spreads and to receive fixed interest at a rate of 3.000% through June 15, 2023. The Company entered into three interest rate swap agreements with an aggregate notional value of $600.0 million related to the 2.250% Notes. These interest rate swaps, which were designated as fair value hedges at inception, were entered into to hedge against changes in fair value of the 2.250% Notes resulting from changes in interest rates. The interest rate swap agreements require the Company to pay interest at a variable interest rate of one-month LIBOR plus applicable spreads and to receive fixed interest at a rate of 2.250% through January 15, 2022. The fair value of the interest rate swap agreements in the U.S. at December 31, 2017 and 2016 is $28.5 million and $24.7 million , respectively, and were included in Other non-current liabilities on the consolidated balance sheet. During the year ended December 31, 2017, the Company recorded a net fair value adjustment of $1.5 million related to interest rate swaps and the change in fair value of debt due to interest rate swaps in Other expense in the consolidated statement of operations. One of the Company’s Colombian subsidiaries is party to an interest rate swap agreement with an aggregate notional value of 70.0 billion COP ( $23.5 million ) with certain of the lenders under the Colombian Credit Facility. The interest rate swap agreement, which was designated as a cash flow hedge at inception, was entered into to manage exposure to variability in interest rates on debt. The interest rate swap agreement requires the payment of a fixed interest rate of 5.74% and pays variable interest at the three-month Inter-bank Rate (IBR) through the earlier of termination of the underlying debt or April 24, 2021. The notional value is reduced in accordance with the repayment schedule under the Colombian Credit Facility. The fair value of the interest rate swap agreements in Colombia at December 31, 2017 was $0.5 million and was included in Other non-current liabilities on the consolidated balance sheet. Embedded Derivative in Lease Agreement In connection with the acquisition of communications sites in Nigeria, the Company entered into a site lease agreement where a portion of the monthly rent to be received is escalated based on an index outside the lessor’s economic environment. The fair value of the portion of the lease tied to the U.S. CPI was $14.6 million at the date of acquisition and was recorded in Notes receivable and other non-current assets on the consolidated balance sheet. The fair value of the Company’s embedded derivative is determined using a discounted cash flow approach, which takes into consideration Level 3 unobservable inputs, including expected future cash flows over the period in which the associated payment is expected to be received and applies a discount factor that captures uncertainties in the future periods associated with the expected payment. During the year ended December 31, 2017, the Company recorded $0.9 million of a fair value adjustment, which was recorded in Other expense in the consolidated statement of operations. Acquisition-Related Contingent Consideration Acquisition-related contingent consideration is initially measured and recorded at fair value as an element of consideration paid in connection with an acquisition with subsequent adjustments recognized in Other operating expenses in the consolidated statements of operations. The fair value of acquisition-related contingent consideration, and any subsequent changes in fair value, is determined by using a discounted probability-weighted approach, which takes into consideration Level 3 unobservable inputs, including assessments of expected future cash flows over the period in which the obligation is expected to be settled, and applies a discount factor that captures the uncertainties associated with the obligation. Changes in the unobservable inputs of Level 3 assets or liabilities could significantly impact the fair value of these assets or liabilities recorded in the accompanying consolidated balance sheets, with the adjustments being recorded in the consolidated statements of operations. As of December 31, 2017 , the Company estimates that the value of all potential acquisition-related contingent consideration required payments to be between $9.1 million and $10.1 million . The changes in fair value of the contingent consideration were as follows during the years ended December 31,: 2017 2016 Balance as of January 1 $ 15.4 $ 12.4 Additions — 8.8 Settlements — (0.3 ) Change in fair value (6.3 ) (6.4 ) Foreign currency translation adjustment 1.0 0.9 Balance as of December 31 $ 10.1 $ 15.4 Items Measured at Fair Value on a Nonrecurring Basis Assets Held and Used —The Company’s long-lived assets are recorded at amortized cost and, if impaired, are adjusted to fair value using Level 3 inputs. During the year ended December 31, 2017 , certain long-lived assets held and used with a carrying value of $21.7 billion were written down to their net realizable value as a result of an asset impairment charge of $ 211.4 million . During the year ended December 31, 2016 , certain long-lived assets held and used with a carrying value of $12.7 billion were written down to their net realizable value as a result of an asset impairment charge of $28.5 million . The asset impairment charges are recorded in Other operating expenses in the accompanying consolidated statements of operations. These adjustments were determined by comparing the estimated fair value utilizing projected future discounted cash flows to be provided from the long-lived assets to the asset’s carrying value. There were no other items measured at fair value on a nonrecurring basis during the year ended December 31, 2017 . Fair Value of Financial Instruments —The Company’s financial instruments for which the carrying value reasonably approximates fair value at December 31, 2017 and 2016 include cash and cash equivalents, restricted cash, accounts receivable and accounts payable. The Company’s estimates of fair value of its long-term obligations, including the current portion, are based primarily upon reported market values. For long-term debt not actively traded, fair value is estimated using either indicative price quotes or a discounted cash flow analysis using rates for debt with similar terms and maturities. As of December 31, 2017 , the carrying value and fair value of long-term obligations, including the current portion, were $20.2 billion and $20.6 billion , respectively, of which $13.3 billion was measured using Level 1 inputs and $7.3 billion was measured using Level 2 inputs. As of December 31, 2016 , the carrying value and fair value of long-term obligations, including the current portion, were $18.5 billion and $18.8 billion , respectively, of which $11.8 billion was measured using Level 1 inputs and $7.0 billion was measured using Level 2 inputs.

INCOME TAXES

INCOME TAXES	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	INCOME TAXES The Company has filed, for prior taxable years through its taxable year ended December 31, 2011, consolidated U.S. federal tax returns, which included all of its then wholly owned domestic subsidiaries. For its taxable year commencing January 1, 2012, the Company filed, and intends to continue to file, as a REIT, and its domestic TRSs filed, and intend to continue to file, separate tax returns as required. The Company also files tax returns in various states and countries. The Company’s state tax returns reflect different combinations of the Company’s subsidiaries and are dependent on the connection each subsidiary has with a particular state and form of organization. The following information pertains to the Company’s income taxes on a consolidated basis. The income tax provision from continuing operations consisted of the following for the years ended December 31,: 2017 2016 2015 Current: Federal $ (0.1 ) $ (26.5 ) $ (73.9 ) State (3.8 ) (2.0 ) (21.2 ) Foreign (113.4 ) (100.1 ) (55.1 ) Deferred: Federal 0.2 (0.6 ) 9.1 State 1.0 (0.3 ) — Foreign 85.4 (26.0 ) (16.9 ) Income tax provision $ (30.7 ) $ (155.5 ) $ (158.0 ) The effective tax rate (“ETR”) on income from continuing operations for the years ended December 31, 2017, 2016 and 2015 differs from the federal statutory rate primarily due to the Company’s qualification for taxation as a REIT, as well as adjustments for foreign items. As a REIT, the Company may deduct earnings distributed to stockholders against the income generated by its REIT operations. In addition, the Company is able to offset certain income by utilizing its NOLs, subject to specified limitations. The Tax Act significantly changes how the U.S. taxes corporations. The Tax Act contains several key provisions including, among other things, a reduction in the corporate income rate from 35% to 21% for tax years beginning after December 31, 2017. As a result of this change in tax rate, the rate at which the Company’s deferred tax assets of the Company’s taxable REIT subsidiaries decreased, resulting in additional tax expense of $2.4 million , which did not significantly impact the Company's effective tax rate. However, the full impact of this change in tax law is provisional and subject to further analysis. In 2015, there was an income tax law change in Ghana that disallowed unused capital allowances to be carried into 2016, which resulted in a charge to income tax expense for the year ended December 31, 2015. In 2017, the Ghana Revenue Authority issued Practice Note Number DT/2016/010 (the “Practice Note”), which clarified the Capital Allowance section of the Income Tax Act of 2015. The Practice Note allowed for unused Capital Allowance from 2015 to be treated as a deduction in 2016. As a result, the Company recorded a tax benefit of $17.8 million for the year ended December 31, 2017. Reconciliation between the U.S. statutory rate and the effective rate from continuing operations is as follows for the years ended December 31: 2017 2016 2015 Statutory tax rate 35 % 35 % 35 % Adjustment to reflect REIT status (1) (35 ) (35 ) (35 ) Foreign taxes 1 5 3 Foreign withholding taxes 3 4 3 Uncertain tax positions — 5 — Changes in tax laws (2 ) — 2 MIPT tax election (2) — — 11 Effective tax rate 2 % 14 % 19 % _______________ (1) As a result of the ability to utilize the dividends paid deduction to offset our REIT income and gains. (2) Includes federal and state taxes, net of federal benefit. The domestic and foreign components of income from continuing operations before income taxes are as follows for the years ended December 31,: 2017 2016 2015 United States $ 971.2 $ 882.6 $ 785.2 Foreign 284.9 243.3 44.8 Total $ 1,256.1 $ 1,125.9 $ 830.0 The components of the net deferred tax asset and liability and related valuation allowance were as follows as of December 31,: 2017 2016 Assets: Net operating loss carryforwards $ 287.0 $ 278.7 Accrued asset retirement obligations 157.0 130.0 Stock-based compensation 3.9 4.3 Unearned revenue 19.3 29.0 Unrealized loss on foreign currency 27.4 26.9 Other accruals and allowances 50.2 45.6 Items not currently deductible and other 28.0 26.9 Liabilities: Depreciation and amortization (1,073.9 ) (942.4 ) Deferred rent (35.9 ) (27.1 ) Other (14.7 ) (9.4 ) Subtotal (551.7 ) (437.5 ) Valuation allowance (142.0 ) (144.4 ) Net deferred tax liabilities $ (693.7 ) $ (581.9 ) Effective January 1, 2016, the Company adopted new guidance on the accounting for share-based payment transactions. As part of this new guidance, excess windfall tax benefits and tax deficiencies related to the Company’s stock option exercises and restricted stock unit vestings are recognized as an income tax benefit or expense in the consolidated statement of operations in the period in which the deduction occurs. Excess windfall tax benefits and tax deficiencies are therefore not anticipated when determining the annual ETR and are instead recognized in the interim period in which those items occur. At December 31, 2017 and 2016, the Company has provided a valuation allowance of $142.0 million and $144.4 million , respectively, which primarily relates to foreign items. During 2017 , the Company decreased the amounts recorded as valuation allowances in certain foreign jurisdictions as the Company believes these deferred tax assets are more likely than not to be realized. The decrease in the valuation allowance for the year ending December 31, 2017, is offset by an increase due to uncertainty as to the timing of, and the Company’s ability to recover, net deferred tax assets in certain foreign operations in the foreseeable future as well as fluctuations in foreign currency exchange rates. The amount of deferred tax assets considered realizable, however, could be adjusted if objective evidence in the form of cumulative losses is no longer present and additional weight may be given to subjective evidence such as the Company’s projections for growth. A summary of the activity in the valuation allowance is as follows: 2017 2016 2015 Balance as of January 1, $ 144.4 $ 137.0 $ 141.2 Additions (1) 11.6 14.1 19.5 Reversals (9.1 ) — — Foreign currency translation (4.9 ) (6.7 ) (23.7 ) Balance as of December 31, $ 142.0 $ 144.4 $ 137.0 _______________ (1) Includes net charges to expense and allowances established through goodwill at acquisition. The recoverability of the Company’s deferred tax assets has been assessed utilizing projections based on its current operations. Accordingly, the recoverability of the deferred tax assets is not dependent on material asset sales or other non-routine transactions. Based on its current outlook of future taxable income during the carryforward period, the Company believes that deferred tax assets, other than those for which a valuation allowance has been recorded, will be realized. The Tax Act requires a mandatory one-time inclusion of accumulated earnings of foreign subsidiaries, and as a result, all previously unremitted earnings for which no U.S. deferred tax liability had been accrued have now been included in the calculation of U.S. taxable income. Notwithstanding the inclusion of these amounts in the determination of U.S. taxable income, the Company intends to continue to invest these foreign earnings indefinitely outside of the U.S. and does not expect to incur any significant, additional taxes, primarily withholding taxes, related to such amounts. At December 31, 2017 , the Company had net federal, state and foreign operating loss carryforwards available to reduce future taxable income. If not utilized, the Company’s NOLs expire as follows: Years ended December 31, Federal State Foreign 2018 to 2022 $ — $ 90.7 $ 73.2 2023 to 2027 — 361.3 192.3 2028 to 2032 141.6 85.9 — 2033 to 2037 24.6 122.7 — Indefinite carryforward — — 739.7 Total $ 166.2 $ 660.6 $ 1,005.2 As of December 31, 2017 and 2016 , the total amount of unrecognized tax benefits that would impact the ETR, if recognized, is $105.8 million and $102.9 million , respectively. The amount of unrecognized tax benefits for the year ended December 31, 2017 includes additions to the Company’s existing tax positions of $7.6 million . The Company expects the unrecognized tax benefits to change over the next 12 months if certain tax matters ultimately settle with the applicable taxing jurisdiction during this timeframe, or if the applicable statute of limitations lapses. The impact of the amount of such changes to previously recorded uncertain tax positions could range from zero to $11.8 million . A reconciliation of the beginning and ending amount of unrecognized tax benefits are as follows for the years ended December 31,: 2017 2016 2015 Balance at January 1 $ 107.6 $ 28.1 $ 31.9 Additions based on tax positions related to the current year 7.6 82.9 5.0 Additions for tax positions of prior years — — — Foreign currency 1.9 (0.2 ) (5.3 ) Reduction as a result of the lapse of statute of limitations and effective settlements (0.4 ) (3.2 ) (3.5 ) Balance at December 31 $ 116.7 $ 107.6 $ 28.1 During the years ended December 31, 2017 , 2016 and 2015 , the statute of limitations on certain unrecognized tax benefits lapsed and certain positions were effectively settled, which resulted in a decrease of $0.4 million , $3.2 million and $3.5 million , respectively, in the liability for uncertain tax benefits, all of which reduced the income tax provision. The Company recorded penalties and tax-related interest expense to the tax provision of $5.0 million , $9.2 million and $3.2 million for the years ended December 31, 2017 , 2016 and 2015 , respectively. In addition, due to the expiration of the statute of limitations in certain jurisdictions, the Company reduced its liability for penalties and income tax-related interest expense related to uncertain tax positions during the years ended December 31, 2017, 2016 and 2015 by $0.6 million , $3.4 million and $3.1 million , respectively. As of December 31, 2017 and 2016 , the total amount of accrued income tax-related interest and penalties included in the consolidated balance sheets were $29.0 million and $24.3 million , respectively. The Company has filed for prior taxable years, and for its taxable year ended December 31, 2017 will file, numerous consolidated and separate income tax returns, including U.S. federal and state tax returns and foreign tax returns. The Company is subject to examination in the U.S. and various state and foreign jurisdictions for certain tax years. As a result of the Company’s ability to carryforward federal, state and foreign NOLs, the applicable tax years generally remain open to examination several years after the applicable loss carryforwards have been used or have expired. The Company regularly assesses the likelihood of additional assessments in each of the tax jurisdictions resulting from these examinations. The Company believes that adequate provisions have been made for income taxes for all periods through December 31, 2017 .

STOCK-BASED COMPENSATION

STOCK-BASED COMPENSATION	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-Based Compensation	STOCK-BASED COMPENSATION Summary of Stock-Based Compensation Plans —The Company maintains equity incentive plans that provide for the grant of stock-based awards to its directors, officers and employees. The 2007 Equity Incentive Plan, as amended (the “2007 Plan”), provides for the grant of non-qualified and incentive stock options, as well as restricted stock units, restricted stock and other stock-based awards. Exercise prices for non-qualified and incentive stock options are not less than the fair value of the underlying common stock on the date of grant. Equity awards typically vest ratably, generally over four years for RSUs and stock options and three years for PSUs. Stock options generally expire 10 years from the date of grant. As of December 31, 2017 , the Company had the ability to grant stock-based awards with respect to an aggregate of 8.5 million shares of common stock under the 2007 Plan. In addition, the Company maintains an employee stock purchase plan (the “ESPP”) pursuant to which eligible employees may purchase shares of the Company’s common stock on the last day of each bi-annual offering period at a 15% discount from the lower of the closing market value on the first or last day of such offering period. The offering periods run from June 1 through November 30 and from December 1 through May 31 of each year. During the years ended December 31, 2017, 2016 and 2015, the Company recorded and capitalized the following stock-based compensation expenses: 2017 2016 2015 Stock-based compensation expense $ 108.5 $ 89.9 $ 90.5 Stock-based compensation expense capitalized as property and equipment 1.6 1.4 2.1 Stock Options —The fair value of each option granted during the period was estimated on the date of grant using the Black-Scholes option pricing model based on the assumptions noted in the table below. The expected life of stock options (estimated period of time outstanding) was estimated using the vesting term and historical exercise behavior of the Company’s employees. The risk-free interest rate was based on the U.S. Treasury yield with a term that approximated the estimated life in effect at the accounting measurement date. The expected volatility of the underlying stock price was based on historical volatility for a period equal to the expected life of the stock options. The expected annual dividend yield was the Company’s best estimate of expected future dividend yield. Key assumptions used to apply this pricing model were as follows: 2017 2016 2015 Range of risk-free interest rate 1.88%-1.94% 1.00%-1.73% 1.32% - 1.62% Weighted average risk-free interest rate 1.89% 1.44% 1.61% Range of expected life of stock options 5.2 years 4.5 - 5.2 years 4.5 years Range of expected volatility of the underlying stock price 18.95% - 19.45% 20.59% - 21.45% 21.09% - 21.24% Weighted average expected volatility of underlying stock price 19.05% 21.43% 21.09% Range of expected annual dividend yield 2.40% 1.85% - 2.40% 1.50% - 1.85% The weighted average grant date fair value per share during the years ended December 31, 2017 , 2016 and 2015 was $16.84 , $14.60 and $15.06 , respectively. The intrinsic value of stock options exercised during the years ended December 31, 2017 , 2016 and 2015 was $100.3 million , $77.6 million and $32.1 million , respectively. As of December 31, 2017 , total unrecognized compensation expense related to unvested stock options was $12.4 million and is expected to be recognized over a weighted average period of approximately two years. The amount of cash received from the exercise of stock options was $110.7 million during the year ended December 31, 2017 . The Company’s option activity for the year ended December 31, 2017 was as follows: Options Weighted Average Exercise Price Weighted Average Remaining Life (Years) Aggregate Intrinsic Value (in millions) Outstanding as of January 1, 2017 7,269,376 $78.00 Granted 6,534 118.20 Exercised (1,663,001 ) 66.57 Forfeited (55,348 ) 93.09 Expired — — Outstanding as of December 31, 2017 5,557,561 $81.32 6.07 $341.0 Exercisable as of December 31, 2017 3,425,213 $74.47 5.24 $233.6 Vested or expected to vest as of December 31, 2017 5,557,561 $81.32 6.07 $341.0 The following table sets forth information regarding options outstanding at December 31, 2017 : Options Outstanding Options Exercisable Outstanding Number of Options Range of Exercise Price Per Share Weighted Average Exercise Price Per Share Weighted Average Remaining Life (Years) Options Exercisable Weighted Average Exercise Price Per Share 593,231 $28.39 - $50.78 $ 45.76 2.59 593,231 $ 45.76 552,500 52.33 - 74.06 61.97 4.17 552,500 61.97 641,460 76.90 - 79.45 76.92 5.15 639,631 76.91 1,242,705 81.18 - 94.23 81.59 6.19 812,744 81.42 2,484,098 94.57 - 94.71 94.62 7.45 820,043 94.59 43,567 96.46 - 121.15 109.38 8.33 7,064 103.90 5,557,561 $28.39 - $121.15 $ 81.32 6.07 3,425,213 $ 74.47 Restricted Stock Units and Performance-Based Restricted Stock Units —The Company’s RSU and PSU activity for the year ended December 31, 2017 was as follows: RSUs Weighted Average Grant Date Fair Value PSUs Weighted Average Grant Date Fair Value Outstanding as of January 1, 2017 (1) 1,663,743 $ 90.76 242,757 $ 93.92 Granted (2) 840,467 114.22 201,274 113.52 Vested (680,610 ) 88.12 — — Forfeited (80,875 ) 101.51 — — Outstanding as of December 31, 2017 1,742,725 $ 102.60 444,031 $ 102.81 Expected to vest as of December 31, 2017 1,742,725 $ 102.60 444,031 $ 102.81 _______________ (1) PSUs represent the shares issuable for the 2015 PSUs (as defined below) at the end of the three -year performance cycle based on achievement against the performance metric for the first and second year’s performance periods, or 73,417 shares, and the target number of shares issuable at the end of the three -year performance period for the 2016 PSUs (as defined below), or 169,340 shares. (2) PSUs represent the shares issuable for the 2015 PSUs at the end of the three-year performance cycle based on exceeding the performance metric for the third year’s performance period, or 46,754 shares, and the target number of shares issuable at the end of the three-year performance cycle for the 2017 PSUs (as defined below), or 154,520 shares. Restricted Stock Units— The total fair value of RSUs that vested during the year ended December 31, 2017 was $78.3 million . As of December 31, 2017 , total unrecognized compensation expense related to unvested RSUs granted under the 2007 Plan was $101.6 million and is expected to be recognized over a weighted average period of approximately two years . Performance-Based Restricted Stock Units— During the years ended December 31, 2017 and 2016, the Company’s Compensation Committee granted an aggregate of 154,520 PSUs (the “2017 PSUs”) and 169,340 PSUs (the “2016 PSUs”), respectively, to its executive officers and established the performance metrics for these awards. During the year ended December 31, 2015, the Company’s Compensation Committee granted an aggregate of 70,135 PSUs to its executive officers (the “2015 PSUs”) and established the performance metric for this award. Threshold, target and maximum parameters were established for the metrics for a three -year performance period with respect to the 2017 PSUs and the 2016 PSUs, and for each year in the three -year performance period with respect to the 2015 PSUs, and will be used to calculate the number of shares that will be issuable when the award vests, which may range from 0% to 200% of the target amounts. At the end of the three -year performance period, the number of shares that vest will depend on the degree of achievement against the pre-established performance goals. PSUs will be paid out in common stock at the end of the performance period, subject generally to the executive’s continued employment. In the event of the executive’s death, disability or qualifying retirement, PSUs will be paid out pro rata in accordance with the terms of the applicable award agreement. PSUs will accrue dividend equivalents prior to vesting, which will be paid out only in respect of shares actually vested. During the year ended December 31, 2017 , the Company recorded $23.6 million in stock-based compensation expense for equity awards in which the performance goals have been established and were probable of being achieved. The remaining unrecognized compensation expense related to these awards at December 31, 2017 , was $22.1 million based on the Company’s current assessment of the probability of achieving the performance goals. The weighted-average period over which the cost will be recognized is approximately two years .

REDEEMABLE NONCONTROLLING INTER

REDEEMABLE NONCONTROLLING INTERESTS	12 Months Ended
	Dec. 31, 2017
Temporary Equity Disclosure [Abstract]
Redeemable Noncontrolling Interests	REDEEMABLE NONCONTROLLING INTERESTS Redeemable Noncontrolling Interests —In connection with the Viom Acquisition, the Company, through one of its subsidiaries, entered into a shareholders agreement (the “Shareholders Agreement”) with Viom and the following remaining Viom shareholders: Tata Sons Limited, Tata Teleservices Limited (“Tata Teleservices”), IDFC Private Equity Fund III, Macquarie SBI Infrastructure Investments Pte Limited and SBI Macquarie Infrastructure Trust (collectively, the “Remaining Shareholders”). The Shareholders Agreement provides for, among other things, put options held by certain of the Remaining Shareholders, which allow the Remaining Shareholders to sell outstanding shares of ATC TIPL, and call options held by the Company, which allow the Company to buy the noncontrolling shares of ATC TIPL. The put options, which are not under the Company’s control, cannot be separated from the noncontrolling interests. As a result, the combination of the noncontrolling interests and the redemption feature require classification as redeemable noncontrolling interests in the consolidated balance sheet, separate from equity. Given the provisions governing the put rights, the redeemable noncontrolling interests are recorded outside of permanent equity at their redemption value. The noncontrolling interests become redeemable after the passage of time, and therefore, the Company records the carrying amount of the noncontrolling interests at the greater of (i) the initial carrying amount, increased or decreased for the noncontrolling interests’ share of net income or loss and foreign currency translation adjustments, or (ii) the redemption value. If required, the Company will adjust the redeemable noncontrolling interests to redemption value on each balance sheet date with changes in redemption value recognized as an adjustment to Distributions in excess of earnings. The put options may be exercised, requiring the Company to purchase the Remaining Shareholders’ equity interests, on specified dates beginning April 1, 2018 through March 31, 2021. The price of the put options will be based on the fair market value of the exercising Remaining Shareholder’s interest in the Company’s India operations at the time the option is exercised. Put options held by certain of the Remaining Shareholders are subject to a floor price of 216 INR per share. The following is a reconciliation of the changes in the Redeemable noncontrolling interests: Balance as of January 1, 2016 $ — Fair value at acquisition 1,100.9 Net income attributable to noncontrolling interests 13.9 Foreign currency translation adjustment attributable to noncontrolling interests (23.5 ) Balance as of January 1, 2017 $ 1,091.3 Net loss attributable to noncontrolling interests (33.4 ) Foreign currency translation adjustment attributable to noncontrolling interests 68.3 Balance as of December 31, 2017 $ 1,126.2

EQUITY

EQUITY	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Equity	EQUITY Series A Preferred Stock — In May 2014, the Company issued 6,000,000 shares of its 5.25% Mandatory Convertible Preferred Stock, Series A, par value $0.01 per share (the “Series A Preferred Stock”). During the year ended December 31, 2017, all outstanding shares of the Series A Preferred Stock converted at a rate of 0.9337 per share into an aggregate of 5,602,153 shares of the Company’s common stock pursuant to the provisions of the Certificate of Designations governing the Series A Preferred Stock. The Company paid cash in lieu of fractional shares of the Company’s common stock. These payments were recorded as a reduction to Additional paid-in capital. On May 15, 2017, the Company paid the final dividend of $7.9 million to holders of the Series A Preferred Stock at the close of business on May 1, 2017. Series B Preferred Stock —In March 2015, the Company issued 1,375,000 shares of its 5.50% Mandatory Convertible Preferred Stock, Series B, par value $0.01 per share (the “Series B Preferred Stock”). As of December 31, 2017, the Company had 13,749,860 depositary shares, each representing a 1/10th interest in a share of its Series B Preferred Stock outstanding, after giving effect to the early conversion of 140 depositary shares at the option of the holder at a conversion rate of 0.8687 per depositary share in May 2017. On February 15, 2018, the Company paid the final dividend of $18.9 million to holders of the Series B Preferred Stock at the close of business on February 1, 2018. Unless converted or redeemed earlier, each share of the Series B Preferred Stock converted automatically on February 15, 2018 at a rate of 8.7420 per share of Series B Preferred Stock, or 0.8742 per depositary share, each representing a 1/10th interest in a share of Series B Preferred Stock, into shares of the Company’s common stock pursuant to the provisions of the Certificate of Designations governing the Series B Preferred Stock. As a result of the conversions of the Series B Preferred Stock in 2018, the Company issued an aggregate of 12,020,064 shares of its common stock. The Company paid cash in lieu of fractional shares of the Company’s common stock. These payments were recorded as a reduction to Additional paid-in capital. Dividends —The Company may pay dividends in cash or, subject to certain limitations, in shares of common stock or any combination of cash and shares of common stock. Sales of Equity Securities —The Company receives proceeds from sales of its equity securities pursuant to the ESPP and upon exercise of stock options granted under its equity incentive plan. During the year ended December 31, 2017, the Company received an aggregate of $119.7 million in proceeds upon exercises of stock options and sales pursuant to the ESPP. Stock Repurchase Programs —In March 2011, the Company’s Board of Directors approved a stock repurchase program, pursuant to which the Company is authorized to repurchase up to $1.5 billion of its common stock (the “2011 Buyback”). In December 2017, the Board of Directors approved an additional stock repurchase program, pursuant to which the Company is authorized to repurchase up to $2.0 billion of its common stock (the “2017 Buyback”). During the year ended December 31, 2017, the Company resumed the 2011 Buyback and repurchased 6,099,150 shares of its common stock thereunder for an aggregate of $766.3 million , including commissions and fees. As of December 31, 2017, the Company had repurchased a total of 12,356,054 shares of its common stock under the 2011 Buyback for an aggregate of $1.2 billion , including commissions and fees. There were no repurchases under the 2017 Buyback. Under each program, the Company is authorized to purchase shares from time to time through open market purchases, in privately negotiated transactions not to exceed market prices, and (with respect to such open market purchases) pursuant to plans adopted in accordance with Rule 10b5-1 under the Securities Exchange Act of 1934 in accordance with securities laws and other legal requirements, and subject to market conditions and other factors. The Company expects to fund any further repurchases of its common stock through a combination of cash on hand, cash generated by operations and borrowings under its credit facilities. Purchases under the 2011 Buyback and 2017 Buyback are subject to the Company having available cash to fund repurchases. Distributions —During the years ended December 31, 2017 , 2016 and 2015, the Company declared the following cash distributions: For the year ended December 31, 2017 2016 2015 Distribution Aggregate Distribution Aggregate Distribution Aggregate Common Stock $ 2.62 $ 1,122.5 $ 2.17 $ 923.7 $ 1.81 $ 766.4 Series A Preferred Stock $ 2.63 $ 15.8 $ 5.25 $ 31.5 $ 3.94 $ 23.7 Series B Preferred Stock $ 55.00 $ 75.6 $ 55.00 $ 75.6 $ 38.65 $ 53.1 The following table characterizes the tax treatment of distributions declared per share of common stock and Mandatory Convertible Preferred Stock. For the year ended December 31, 2017 2016 2015 Per Share % Per Share % Per Share % Common Stock Ordinary dividend $ 2.6200 (1) 100.00 % $ 2.1700 100.00 % $ 1.2694 70.13 % Capital gains distribution — — — — 0.5406 29.87 Total $ 2.6200 100.00 % $ 2.1700 100.00 % $ 1.8100 100.00 % Series A Preferred Stock Ordinary dividend $ 3.3643 (2) 100.00 % $ 6.4578 (3) 100.00 % $ 3.6818 (4) 70.13 % Capital gains distribution — — — — 1.5682 29.87 Total $ 3.3643 100.00 % $ 6.4578 100.00 % $ 5.2500 100.00 % Series B Preferred Stock (5) Ordinary dividend $ 6.5233 (6) 100.00 % $ 5.5000 100.00 % $ 2.7107 70.13 % Capital gains distribution — — — — 1.1546 29.87 Total $ 6.5233 100.00 % $ 5.5000 100.00 % $ 3.8653 100.00 % _______________ (1) Includes dividend declared on December 6, 2017 of $0.70 per share, which was paid on January 16, 2018 to common stockholders of record at the close of business on December 28, 2017. (2) Includes a deemed distribution as a result of a conversion rate adjustment triggered on April 27, 2017. (3) Includes a deemed distribution as a result of a conversion rate adjustment triggered on June 17, 2016. (4) Includes dividend declared on December 2, 2014 of $1.3125 per share, which was paid on February 16, 2015 to preferred stockholders of record at the close of business on February 1, 2015. (5) Represents the tax treatment on dividends per depositary share, each of which represents a 1/10th interest in a share of Series B Preferred Stock. (6) Includes a deemed distribution as a result of a conversion rate adjustment triggered on April 12, 2017. The Company accrues distributions on unvested restricted stock units, which are payable upon vesting. As of December 31, 2017 , the amount accrued for distributions payable related to unvested restricted stock units was $10.1 million . During the year ended December 31, 2017 , the Company paid $3.0 million of distributions payable upon the vesting of restricted stock units. To maintain its qualification for taxation as a REIT, the Company expects to continue paying distributions, the amount, timing and frequency of which will be determined and subject to adjustment by the Company’s Board of Directors.

IMPAIRMENTS, NET LOSS ON SALE O

IMPAIRMENTS, NET LOSS ON SALE OF LONG-LIVED ASSETS	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment Impairment or Disposal [Abstract]
Impairments, Net Loss on Sale of Long-lived Assets	IMPAIRMENTS, NET LOSS ON SALES OF LONG-LIVED ASSETS During the years ended December 31, 2017 , 2016 and 2015 , the Company recorded impairment charges and net losses on sales or disposals of long-lived assets of $244.2 million , $53.6 million and $29.8 million , respectively. These charges were primarily related to assets included in the Company’s Asia property segment for the year ended December 31, 2017 and the U.S. property segment for the years ended December 31, 2016 and 2015, and are included in Other operating expenses in the consolidated statements of operations. Included in these amounts were impairment charges of $211.4 million , $28.5 million and $15.1 million for the years ended December 31, 2017 , 2016 and 2015 , respectively, to write down certain assets to their net realizable value after an indicator of impairment was identified. These assets consisted primarily of towers, which are assessed on an individual basis, network location intangibles, which relate directly to towers, and tenant-related intangibles. For the year ended December 31, 2017 impairment charges included $81.0 million related to tower and network intangible assets and $100.1 million related to tenant relationships due primarily to carrier consolidation in the Company’s Asia property segment. For the years ended December 31, 2017 and 2016, impairment charges included amounts related to land easements. Also included in these amounts were net losses associated with the sale or disposal of certain non-core towers, other assets and other miscellaneous items of $32.8 million , $25.1 million and $14.7 million for the years ended December 31, 2017 , 2016 and 2015 , respectively. In October 2017, one of the Company’s tenants in Asia, Tata Teleservices, informed the Department of Telecommunications in India of its intent to exit the wireless telecommunications business and announced plans to transfer its business to another telecommunications provider. The Company considered the recent developments regarding these events when conducting its annual impairment test for the Tata Teleservices tenant relationship, which did not result in an impairment since the estimated probability-weighted undiscounted cash flows were in excess of the carrying value of this asset. However, the Company will continue to monitor the status of these developments, as it is possible that the estimated future cash flows may differ from current estimates. Changes in estimated cash flows from Tata Teleservices could have an impact on previously recorded tangible and intangible assets, including amounts originally recorded as tenant-related intangibles, which have a current net book value of $436.4 million .

EARNINGS PER COMMON SHARE

EARNINGS PER COMMON SHARE	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Earnings Per Common Share	EARNINGS PER COMMON SHARE The following table sets forth basic and diluted net income per common share computational data for the years ended December 31, (shares in thousands, except per share data): 2017 2016 2015 Net income attributable to American Tower Corporation stockholders $ 1,238.9 $ 956.4 $ 685.1 Dividends on preferred stock (87.4 ) (107.1 ) (90.2 ) Net income attributable to American Tower Corporation common stockholders 1,151.5 849.3 594.9 Basic weighted average common shares outstanding 428,181 425,143 418,907 Dilutive securities 3,507 4,140 4,108 Diluted weighted average common shares outstanding 431,688 429,283 423,015 Basic net income attributable to American Tower Corporation common stockholders per common share $ 2.69 $ 2.00 $ 1.42 Diluted net income attributable to American Tower Corporation common stockholders per common share $ 2.67 $ 1.98 $ 1.41 Shares Excluded From Dilutive Effect The following shares were not included in the computation of diluted earnings per share because the effect would be anti-dilutive for the years ended December 31, (in thousands, on a weighted average basis): 2017 2016 2015 Restricted stock awards 3 6 — Stock options 4 817 1,606 Preferred stock 14,040 17,509 15,408

COMMITMENTS AND CONTINGENCIES

COMMITMENTS AND CONTINGENCIES	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	COMMITMENTS AND CONTINGENCIES Litigation —The Company periodically becomes involved in various claims, lawsuits and proceedings that are incidental to its business. In the opinion of Company management, after consultation with counsel, there are no matters currently pending that would, in the event of an adverse outcome, materially impact the Company’s consolidated financial position, results of operations or liquidity. Verizon Transaction —On March 27, 2015, the Company entered into an agreement with various operating entities of Verizon that provides for the lease, sublease or management of approximately 11,300 wireless communications sites from Verizon commencing March 27, 2015. The average term of the lease or sublease for all sites at the inception of the agreement was approximately 28 years , assuming renewals or extensions of the underlying ground leases for the sites. The Company has the option to purchase the leased sites in tranches, subject to the applicable lease, sublease or management right upon its scheduled expiration. Each tower is assigned to an annual tranche, ranging from 2034 to 2047, which represents the outside expiration date for the sublease rights to the towers in each tranche. The purchase price for each tranche is a fixed amount stated in the lease for such tranche plus the fair market value of certain alterations made to the related towers. The aggregate purchase option price for the towers leased and subleased is approximately $5.0 billion . Verizon will occupy the sites as a tenant for an initial term of ten years with eight optional successive five -year terms; each such term shall be governed by standard master lease agreement terms established as a part of the transaction. AT&T Transaction —The Company has an agreement with SBC Communications Inc., a predecessor entity to AT&T Inc. (“AT&T”), that currently provides for the lease or sublease of approximately 2,350 towers from AT&T with the lease commencing between December 2000 and August 2004. Substantially all of the towers are part of the 2013 Securitization. The average term of the lease or sublease for all sites at the inception of the agreement was approximately 27 years , assuming renewals or extensions of the underlying ground leases for the sites. The Company has the option to purchase the sites subject to the applicable lease or sublease upon its expiration. Each tower is assigned to an annual tranche, ranging from 2013 to 2032, which represents the outside expiration date for the sublease rights to that tower. The purchase price for each site is a fixed amount stated in the lease for that site plus the fair market value of certain alterations made to the related tower by AT&T. As of December 31, 2017 , the Company has purchased an aggregate of 88 of the subleased towers upon expiration of the applicable agreement. The aggregate purchase option price for the remaining towers leased and subleased is $831.3 million and will accrete at a rate of 10% per annum through the applicable expiration of the lease or sublease of a site. For all such sites purchased by the Company prior to June 30, 2020, AT&T will continue to lease the reserved space at the then-current monthly fee which shall escalate in accordance with the standard master lease agreement for the remainder of AT&T’s tenancy. Thereafter, AT&T shall have the right to renew such lease for up to four successive five -year terms. For all such sites purchased by the Company subsequent to June 30, 2020, AT&T has the right to continue to lease the reserved space for successive one -year terms at a rent equal to the lesser of the agreed upon market rate and the then-current monthly fee, which is subject to an annual increase based on changes in the U.S. Consumer Price Index. Alltel Transaction —In December 2000, the Company entered into an agreement with Alltel to acquire towers through a 15 -year sublease agreement. Pursuant to the agreement, as amended, with Verizon Wireless, the Company acquired rights to approximately 1,800 towers in tranches between April 2001 and March 2002. The Company has the option to purchase each tower at the expiration of the applicable sublease. The Company exercised the purchase options for 1,523 towers in a single closing which occurred on December 8, 2016. The Company has provided notice to the tower owner of its intent to exercise the purchase options related to the 243 remaining towers. As of December 31, 2017 , the purchase price per tower was $42,844 payable in cash or, at the tower owner’s option, with 769 shares of the Company’s common stock per tower. The aggregate cash purchase option price for the remaining subleased towers was $10.4 million as of December 31, 2017 . Other Contingencies —The Company is subject to income tax and other taxes in the geographic areas where it operates, and periodically receives notifications of audits, assessments or other actions by taxing authorities. In certain jurisdictions, taxing authorities may issue preliminary notices or assessments while audits are being conducted. These preliminary notices or assessments do not represent amounts that the Company is obligated to pay and are often not reflective of the actual tax liability for which the Company will ultimately be liable. The Company evaluates the circumstances of each notification or assessment based on the information available and records a liability for any potential outcome that is probable or more likely than not unfavorable if the liability is also reasonably estimable. On December 5, 2016, the Company received an income tax assessment of Essar Telecom Infrastructure Private Limited (“ETIPL”) from the India Income Tax Department (the “Tax Department”) for the fiscal year ending 2008 in the amount of 4.75 billion INR ( $69.8 million on the date of assessment) related to capital contributions. The Company challenged the assessment before the Office of Commissioner of Income Tax - Appeals, which ruled in the Company’s favor in January 2018. However, the Tax Department may appeal this ruling at a higher appellate authority. The Company estimates that there is a more likely than not probability that the Company’s position will be sustained upon appeal. Accordingly, no liability has been recorded. Additionally, the assessment was made with respect to transactions that took place in the tax year commencing in 2007, prior to the Company’s acquisition of ETIPL. Under the Company’s definitive acquisition agreement of ETIPL, the seller is obligated to indemnify and defend the Company with respect to any tax-related liability that may arise from activities prior to March 31, 2010. Lease Obligations —The Company leases certain land, office and tower space under operating leases that expire over various terms. Many of the leases contain renewal options with specified increases in lease payments upon exercise of the renewal option. Escalation clauses present in operating leases, excluding those tied to CPI or other inflation-based indices, are recognized on a straight-line basis over the non-cancellable term of the leases. Future minimum rental payments under non-cancellable operating leases include payments for certain renewal periods at the Company’s option because failure to renew could result in a loss of the applicable communications sites and related revenues from tenant leases, thereby making it reasonably assured that the Company will renew the leases. Such payments at December 31, 2017 are as follows: Year Ending December 31, 2018 $ 924 2019 887 2020 848 2021 811 2022 768 Thereafter 6,533 Total $ 10,771 Aggregate rent expense (including the effect of straight-line rent expense) under operating leases for the years ended December 31, 2017 , 2016 and 2015 approximated $1,088.0 million , $986.2 million and $804.8 million , respectively. Future minimum payments under capital leases in effect at December 31, 2017 were as follows: Year Ending December 31, 2018 $ 34 2019 31 2020 26 2021 21 2022 18 Thereafter 166 Total minimum lease payments 296 Less amounts representing interest (130 ) Present value of capital lease obligations $ 166 Tenant Leases —The Company’s lease agreements with its tenants vary depending upon the region and the industry of the tenant, and generally have initial terms of ten years with multiple renewal terms at the option of the tenant. Future minimum rental receipts expected from tenants under non-cancellable operating lease agreements in effect at December 31, 2017 were as follows: Year Ending December 31, 2018 $ 4,927 2019 4,683 2020 4,393 2021 3,957 2022 3,095 Thereafter 11,180 Total $ 32,235 Guaranties and Indemnifications —The Company enters into agreements from time to time in the ordinary course of business pursuant to which it agrees to guarantee or indemnify third parties for certain claims. The Company has also entered into purchase and sale agreements relating to the sale or acquisition of assets containing customary indemnification provisions. The Company’s indemnification obligations under these agreements generally are limited solely to damages resulting from breaches of representations and warranties or covenants under the applicable agreements. In addition, payments under such indemnification clauses are generally conditioned on the other party making a claim that is subject to whatever defenses the Company may have and are governed by dispute resolution procedures specified in the particular agreement. Further, the Company’s obligations under these agreements may be limited in duration and amount, and in some instances, the Company may have recourse against third parties for payments made by the Company. The Company has not historically made any material payments under these agreements and, as of December 31, 2017 , is not aware of any agreements that could result in a material payment.

SUPPLEMENTAL CASH FLOW INFORMAT

SUPPLEMENTAL CASH FLOW INFORMATION	12 Months Ended
	Dec. 31, 2017
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information	SUPPLEMENTAL CASH FLOW INFORMATION Supplemental cash flow information and non-cash investing and financing activities are as follows for the years ended December 31,: 2017 2016 2015 Supplemental cash flow information: Cash paid for interest $ 712.1 $ 645.1 $ 578.0 Cash paid for income taxes (net of refunds of $20.7, $19.6 and $7.1, respectively) 136.5 96.2 157.1 Non-cash investing and financing activities: Increase (decrease) in accounts payable and accrued expenses for purchases of property and equipment and construction activities 34.0 (19.0 ) 2.8 Purchases of property and equipment under capital leases 54.8 55.6 36.9 Fair value of debt assumed through acquisitions — 786.9 — Exercise of purchase option for property and equipment for common shares issued — 120.8 — Settlement of accounts receivable related to acquisitions — — 0.9 Conversion of third-party debt to equity 48.2 — —

BUSINESS SEGMENTS

BUSINESS SEGMENTS	12 Months Ended
	Dec. 31, 2017
Segment Reporting, Measurement Disclosures [Abstract]
Business Segments	BUSINESS SEGMENTS The Company’s primary business is leasing space on multitenant communications sites to wireless service providers, radio and television broadcast companies, wireless data providers, government agencies and municipalities and tenants in a number of other industries. This business is referred to as the Company’s property operations, which as of December 31, 2017, consisted of the following: • U.S.: property operations in the United States; • Asia: property operations in India; • Europe, Middle East and Africa (“EMEA”): property operations in France, Germany, Ghana, Nigeria, South Africa and Uganda; and • Latin America: property operations in Argentina, Brazil, Chile, Colombia, Costa Rica, Mexico, Paraguay and Peru. The Company has applied the aggregation criteria to operations within the EMEA and Latin America property operating segments on a basis that is consistent with management’s review of information and performance evaluations of these regions. The Company’s services segment offers tower-related services in the United States, including site acquisition, zoning and permitting and structural analysis, which primarily support its site leasing business, including the addition of new tenants and equipment on its sites. The services segment is a strategic business unit that offers different services from, and requires different resources, skill sets and marketing strategies than, the property operating segments. The accounting policies applied in compiling segment information below are similar to those described in note 1. Among other factors, in evaluating financial performance in each business segment, management uses segment gross margin and segment operating profit. The Company defines segment gross margin as segment revenue less segment operating expenses excluding stock-based compensation expense recorded in costs of operations; Depreciation, amortization and accretion; Selling, general, administrative and development expense; and Other operating expenses. The Company defines segment operating profit as segment gross margin less Selling, general, administrative and development expense attributable to the segment, excluding stock-based compensation expense and corporate expenses. For reporting purposes, the Latin America property segment gross margin and segment operating profit also include Interest income, TV Azteca, net. These measures of segment gross margin and segment operating profit are also before Interest income, Interest expense, Gain (loss) on retirement of long-term obligations, Other income (expense), Net income (loss) attributable to noncontrolling interests and Income tax benefit (provision). The categories of expenses indicated above, such as depreciation, have been excluded from segment operating performance as they are not considered in the review of information or the evaluation of results by management. There are no significant revenues resulting from transactions between the Company’s operating segments. All intercompany transactions are eliminated to reconcile segment results and assets to the consolidated statements of operations and consolidated balance sheets. Summarized financial information concerning the Company’s reportable segments for the years ended December 31, 2017 , 2016 and 2015 is shown in the following tables. The “Other” column (i) represents amounts excluded from specific segments, such as business development operations, stock-based compensation expense and corporate expenses included in Selling, general, administrative and development expense; Other operating expenses; Interest income; Interest expense; Gain (loss) on retirement of long-term obligations; and Other income (expense), as the amounts are not utilized in assessing each segment’s performance, and (ii) reconciles segment operating profit to Income from continuing operations before income taxes. Property Total Property Services Other Total Year ended December 31, 2017 U.S. Asia EMEA Latin America Segment revenues $ 3,605.7 $ 1,164.4 $ 626.2 $ 1,169.6 $ 6,565.9 $ 98.0 $ 6,663.9 Segment operating expenses (1) 746.5 649.0 238.3 386.1 2,019.9 33.8 2,053.7 Interest income, TV Azteca, net — — — 10.8 10.8 — 10.8 Segment gross margin 2,859.2 515.4 387.9 794.3 4,556.8 64.2 4,621.0 Segment selling, general, administrative and development expense (1) 151.4 82.4 67.9 77.5 379.2 13.7 392.9 Segment operating profit $ 2,707.8 $ 433.0 $ 320.0 $ 716.8 $ 4,177.6 $ 50.5 $ 4,228.1 Stock-based compensation expense $ 108.5 108.5 Other selling, general, administrative and development expense 138.5 138.5 Depreciation, amortization and accretion 1,715.9 1,715.9 Other expense (2) 1,009.1 1,009.1 Income from continuing operations before income taxes $ 1,256.1 Capital expenditures (3) $ 360.6 $ 118.0 $ 141.7 $ 197.4 $ 817.7 $ — $ 17.7 $ 835.4 _______________ (1) Segment operating expenses and segment selling, general, administrative and development expenses exclude stock-based compensation expense of $2.9 million and $105.6 million , respectively. (2) Primarily includes interest expense. (3) Includes $31.8 million of capital lease payments included in Repayments of notes payable, credit facilities, term loan, senior notes and capital leases in the cash flow from financing activities in the Company’s consolidated statement of cash flows. Property Total Property Services Other Total Year ended December 31, 2016 U.S. Asia EMEA Latin America Segment revenues $ 3,370.1 $ 827.6 $ 529.5 $ 985.9 $ 5,713.1 $ 72.6 $ 5,785.7 Segment operating expenses (1) 733.4 465.9 223.7 338.0 1,761.0 27.0 1,788.0 Interest income, TV Azteca, net — — — 10.9 10.9 — 10.9 Segment gross margin 2,636.7 361.7 305.8 658.8 3,963.0 45.6 4,008.6 Segment selling, general, administrative and development expense (1) 147.6 48.2 60.9 60.7 317.4 12.5 329.9 Segment operating profit $ 2,489.1 $ 313.5 $ 244.9 $ 598.1 $ 3,645.6 $ 33.1 $ 3,678.7 Stock-based compensation expense $ 89.9 89.9 Other selling, general, administrative and development expense 126.0 126.0 Depreciation, amortization and accretion 1,525.6 1,525.6 Other expense (2) 811.3 811.3 Income from continuing operations before income taxes $ 1,125.9 Capital expenditures (3) $ 310.7 $ 115.5 $ 86.1 $ 172.6 $ 684.9 $ — $ 16.5 $ 701.4 _______________ (1) Segment operating expenses and segment selling, general, administrative and development expenses exclude stock-based compensation expense of $2.4 million and $87.5 million , respectively. (2) Primarily includes interest expense. (3) Includes $18.9 million of capital lease payments included in Repayments of notes payable, credit facilities, term loan, senior notes and capital leases in the cash flow from financing activities in the Company’s consolidated statement of cash flows. Property Total Property Other Total Year ended December 31, 2015 U.S. Asia EMEA Latin America Segment revenues $ 3,157.5 $ 242.2 $ 395.1 $ 885.6 $ 4,680.4 $ 91.1 $ 4,771.5 Segment operating expenses (1) 678.5 126.9 163.8 304.6 1,273.8 33.0 1,306.8 Interest income, TV Azteca, net — — — 11.2 11.2 — 11.2 Segment gross margin 2,479.0 115.3 231.3 592.2 3,417.8 58.1 3,475.9 Segment selling, general, administrative and development expense (1) 138.6 22.7 48.7 62.2 272.2 15.7 287.9 Segment operating profit $ 2,340.4 $ 92.6 $ 182.6 $ 530.0 $ 3,145.6 $ 42.4 $ 3,188.0 Stock-based compensation expense $ 90.5 90.5 Other selling, general, administrative and development expense 121.4 121.4 Depreciation, amortization and accretion 1,285.3 1,285.3 Other expense (2) 860.8 860.8 Income from continuing operations before income taxes $ 830.0 Capital expenditures $ 367.7 $ 75.4 $ 66.6 $ 201.8 $ 711.5 $ — $ 17.3 $ 728.8 _______________ (1) Segment operating expenses and segment selling, general, administrative and development expenses exclude stock-based compensation expense of $2.0 million and $88.5 million , respectively. (2) Primarily includes interest expense. Additional information relating to the total assets of the Company’s operating segments is as follows for the years ended December 31,: 2017 2016 2015 U.S. property $ 19,032.6 $ 18,846.9 $ 19,286.5 Asia property (1) 4,770.8 4,535.3 736.1 EMEA property (1) 3,213.6 2,062.4 2,249.6 Latin America property (1) 5,868.4 4,938.1 4,401.3 Services 42.3 48.3 68.4 Other (2) 286.6 448.2 162.4 Total assets $ 33,214.3 $ 30,879.2 $ 26,904.3 _______________ (1) Balances are translated at the applicable period end exchange rate, which may impact comparability between periods. (2) Balances include corporate assets such as cash and cash equivalents, certain tangible and intangible assets and income tax accounts that have not been allocated to specific segments. Summarized geographic information related to the Company’s operating revenues for the years ended December 31, 2017 , 2016 and 2015 and long-lived assets as of December 31, 2017 and 2016 is as follows: 2017 2016 2015 Operating Revenues: United States $ 3,703.7 $ 3,442.7 $ 3,248.6 Asia (1): India 1,164.4 827.6 242.2 EMEA (1): France 59.5 — — Germany 63.1 60.2 56.0 Ghana 122.9 116.2 94.5 Nigeria 213.9 215.4 109.7 South Africa 106.5 80.0 80.5 Uganda 60.3 57.7 54.4 Latin America (1): Argentina 15.9 1.0 — Brazil 620.1 506.2 408.6 Chile 40.4 33.8 29.7 Colombia 89.3 79.7 78.4 Costa Rica 19.4 19.0 17.2 Mexico 364.3 331.2 340.5 Paraguay 2.7 — — Peru 17.5 15.0 11.2 Total International 2,960.2 2,343.0 1,522.9 Total operating revenues $ 6,663.9 $ 5,785.7 $ 4,771.5 _______________ (1) Balances are translated at the applicable exchange rate, which may impact comparability between periods. 2017 2016 Long-Lived Assets (1): United States $ 16,930.2 $ 16,969.6 Asia (2): India 4,052.6 4,094.2 EMEA (2): France 1,009.6 — Germany 428.0 397.3 Ghana 171.4 192.2 Nigeria 587.2 640.6 South Africa 330.4 271.8 Uganda 136.9 141.5 Latin America (2): Argentina 117.9 137.6 Brazil 2,557.4 2,626.4 Chile 151.2 137.2 Colombia 369.0 272.3 Costa Rica 112.9 117.5 Mexico 1,396.8 797.8 Paraguay 77.5 — Peru 93.7 66.6 Total International 11,592.5 9,893.0 Total long-lived assets $ 28,522.7 $ 26,862.6 _______________ (1) Includes Property and equipment, net, Goodwill and Other intangible assets, net. (2) Balances are translated at the applicable period end exchange rate, which may impact comparability between periods. The following tenants within the property and services segments individually accounted for 10% or more of the Company’s consolidated operating revenues for the years ended December 31, is as follows: 2017 2016 2015 AT&T 19 % 21 % 24 % Verizon Wireless 16 % 15 % 16 % Sprint 9 % 11 % 13 % T-Mobile 9 % 9 % 10 %

RELATED PARTY TRANSACTIONS

RELATED PARTY TRANSACTIONS	12 Months Ended
	Dec. 31, 2017
Related Party Transaction, Due from (to) Related Party [Abstract]
Related Party Transactions	RELATED PARTY TRANSACTIONS During the years ended December 31, 2017 , 2016 and 2015 , the Company had no significant related party transactions.

SELECTED QUARTERLY FINANCIAL DA

SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED)	12 Months Ended
	Dec. 31, 2017
Selected Quarterly Financial Information [Abstract]
Selected Quarterly Financial Data (Unaudited)	SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) Selected quarterly financial data for the years ended December 31, 2017 and 2016 is as follows (in millions, except per share data): Three Months Ended Year Ended December 31, March 31, June 30, September 30, December 31, 2017: Operating revenues $ 1,616.2 $ 1,662.5 $ 1,680.7 $ 1,704.5 $ 6,663.9 Costs of operations (1) 492.7 517.2 519.8 526.9 2,056.6 Operating income 531.4 576.9 561.1 329.0 1,998.4 Net income 307.4 388.5 334.7 194.8 1,225.4 Net income attributable to American Tower Corporation stockholders 316.1 367.0 317.3 238.5 1,238.9 Dividends on preferred stock (26.8 ) (22.8 ) (18.9 ) (18.9 ) (87.4 ) Net income attributable to American Tower Corporation common stockholders 289.3 344.2 298.4 219.6 1,151.5 Basic net income per share attributable to American Tower Corporation common stockholders 0.68 0.81 0.70 0.51 2.69 Diluted net income per share attributable to American Tower Corporation common stockholders 0.67 0.80 0.69 0.51 2.67 Three Months Ended Year Ended December 31, (2) March 31, June 30, September 30, December 31, 2016: Operating revenues $ 1,289.0 $ 1,442.2 $ 1,514.8 $ 1,539.5 $ 5,785.7 Costs of operations (1) 351.4 459.7 491.2 488.0 1,790.4 Operating income 451.9 432.8 479.1 489.3 1,853.0 Net income 281.3 192.5 263.7 232.9 970.4 Net income attributable to American Tower Corporation stockholders 275.2 187.6 264.5 229.2 956.4 Dividends on preferred stock (26.8 ) (26.8 ) (26.8 ) (26.8 ) (107.1 ) Net income attributable to American Tower Corporation common stockholders 248.4 160.8 237.7 202.4 849.3 Basic net income per share attributable to American Tower Corporation common stockholders 0.59 0.38 0.56 0.48 2.00 Diluted net income per share attributable to American Tower Corporation common stockholders 0.58 0.37 0.55 0.47 1.98 _______________ (1) Represents Operating expenses, exclusive of Depreciation, amortization and accretion, Selling, general, administrative and development expense, and Other operating expenses. (2) The amounts reported for the year ended December 31, 2016 differ slightly from the sum of the quarters due to rounding.

SUBSEQUENT EVENTS

SUBSEQUENT EVENTS	12 Months Ended
	Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	SUBSEQUENT EVENTS [To Be Updated]-

Schedule III - SCHEDULE OF REAL

Schedule III - SCHEDULE OF REAL ESTATE AND ACCUMULATED DEPRECIATION	12 Months Ended
	Dec. 31, 2017
SEC Schedule III, Real Estate and Accumulated Depreciation Disclosure [Abstract]
Real Estate and Accumulated Depreciation Disclosure	AMERICAN TOWER CORPORATION AND SUBSIDIARIES SCHEDULE III—SCHEDULE OF REAL ESTATE AND ACCUMULATED DEPRECIATION (dollars in millions) Description Encumbrances Initial cost to company Cost capitalized subsequent to acquisition Gross amount carried at close of current period Accumulated depreciation at close of current period Date of construction Date acquired Life on which depreciation in latest income statements is computed 149,246 sites (1) $ 3,435.3 (2) (3) (3) $ 15,349.0 (4) $ (5,181.2 ) Various Various Up to 20 years _______________ (1) No single site exceeds 5% of the total amounts indicated in the table above. (2) Certain assets secure debt of $3.4 billion . (3) The Company has omitted this information, as it would be impracticable to compile such information on a site-by-site basis. (4) Does not include those sites under construction. 2017 2016 2015 Gross amount at beginning $ 14,277.0 $ 13,046.3 $ 10,434.3 (1) Additions during period: Acquisitions 499.7 787.2 2,620.8 Discretionary capital projects (2) 120.7 105.3 210.4 Discretionary ground lease purchases (3) 150.4 168.1 144.7 Redevelopment capital expenditures (4) 138.8 136.8 114.1 Capital improvements (5) 65.6 81.8 42.4 Start-up capital expenditures (6) 158.1 128.7 35.6 Other (7) 106.4 139.4 201.1 Total additions 1,239.7 1,547.3 3,369.1 Deductions during period: Cost of real estate sold or disposed (246.5 ) (85.8 ) (61.0 ) Other (8) 78.8 (230.8 ) (696.1 ) Total deductions: (167.7 ) (316.6 ) (757.1 ) Balance at end $ 15,349.0 $ 14,277.0 $ 13,046.3 2017 2016 2015 Gross amount of accumulated depreciation at beginning $ (4,548.1 ) $ (3,994.9 ) $ (3,613.1 ) Additions during period: Depreciation (718.7 ) (647.9 ) (557.1 ) Other — — — Total additions (718.7 ) (647.9 ) (557.1 ) Deductions during period: Amount of accumulated depreciation for assets sold or disposed 100.7 24.9 30.1 Other (8) (15.1 ) 69.8 145.2 Total deductions 85.6 94.7 175.3 Balance at end $ (5,181.2 ) $ (4,548.1 ) $ (3,994.9 ) _______________ (1) Beginning balance has been revised to reflect purchase accounting measurement period adjustments. (2) Includes amounts incurred primarily for the construction of new sites. (3) Includes amounts incurred to purchase or otherwise secure the land under communications sites. (4) Includes amounts incurred to increase the capacity of existing sites, which results in new incremental tenant revenue. (5) Includes amounts incurred to enhance existing sites by adding additional functionality, capacity or general asset improvements. (6) Includes amounts incurred in connection with acquisitions or new market launches. Start-up capital expenditures includes non-recurring expenditures contemplated in acquisitions or new market launch business cases. (7) Primarily includes regional improvements and other additions. (8) Primarily includes foreign currency exchange rate fluctuations and other deductions.

OTHER NON-CURRENT LIABILITIES

OTHER NON-CURRENT LIABILITIES	12 Months Ended
	Dec. 31, 2017
Other Liabilities, Noncurrent [Abstract]
Other Non-Current Liabilities	OTHER NON-CURRENT LIABILITIES Other non-current liabilities consisted of the following as of December 31,: 2017 2016 Unearned revenue $ 509.2 $ 457.3 Deferred rent liability 467.0 407.2 Other miscellaneous liabilities 268.0 278.1 Other non-current liabilities $ 1,244.2 $ 1,142.6

BUSINESS AND SUMMARY OF SIGNI34

BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	12 Months Ended
	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Business	Business —American Tower Corporation (together with its subsidiaries, “ATC” or the “Company”) is one of the largest global real estate investment trusts and a leading independent owner, operator and developer of multitenant communications real estate. The Company’s primary business is the leasing of space on communications sites to wireless service providers, radio and television broadcast companies, wireless data providers, government agencies and municipalities and tenants in a number of other industries. The Company refers to this business as its property operations. Additionally, the Company offers tower-related services in the United States, which the Company refers to as its services operations. These services include site acquisition, zoning and permitting and structural analysis, which primarily support the Company’s site leasing business, including the addition of new tenants and equipment on its sites. The Company’s portfolio primarily consists of towers that it owns and towers that it operates pursuant to long-term lease arrangements, as well as distributed antenna system (“DAS”) networks, which provide seamless coverage solutions in certain in-building and certain outdoor wireless environments. In addition to the communications sites in its portfolio, the Company manages rooftop and tower sites for property owners under various contractual arrangements. The Company also holds other telecommunications infrastructure, including fiber, concrete poles and other assets, and property interests that it leases to communications service providers and third-party tower operators. American Tower Corporation is a holding company that conducts its operations through its directly and indirectly owned subsidiaries and its joint ventures. ATC’s principal domestic operating subsidiaries are American Towers LLC and SpectraSite Communications, LLC. ATC conducts its international operations primarily through its subsidiary, American Tower International, Inc., which in turn conducts operations through its various international holding and operating subsidiaries and joint ventures. The Company operates as a real estate investment trust for U.S. federal income tax purposes (“REIT”). Accordingly, the Company generally is not subject to U.S. federal income taxes on income generated by its REIT operations, including the income derived from leasing space on its towers, as it receives a dividends paid deduction for distributions to stockholders that generally offsets its REIT income and gains. However, the Company remains obligated to pay U.S. federal income taxes on earnings from its domestic taxable REIT subsidiaries (“TRSs”). In addition, the Company’s international assets and operations, regardless of their classification for U.S. tax purposes, continue to be subject to taxation in the foreign jurisdictions where those assets are held or those operations are conducted. The use of TRSs enables the Company to continue to engage in certain businesses while complying with REIT qualification requirements. The Company may, from time to time, change the election of previously designated TRSs to be included as part of the REIT. As of December 31, 2017 , the Company’s REIT qualified businesses included its U.S. tower leasing business, most of its operations in Costa Rica and Mexico, a majority of its operations in Germany and a majority of its indoor DAS networks business and services segment.
Principles of Consolidation and Basis of Presentation	Principles of Consolidation and Basis of Presentation —The accompanying consolidated and condensed consolidated financial statements include the accounts of the Company and those entities in which it has a controlling interest. Investments in entities that the Company does not control are accounted for using the equity or cost method, depending upon the Company’s ability to exercise significant influence over operating and financial policies. All intercompany accounts and transactions have been eliminated. As of December 31, 2017, the Company holds (i) a 51% controlling interest, and MTN Group Limited holds a 49% noncontrolling interest, in each of two joint ventures, one in Ghana and one in Uganda, (ii) a 51% controlling interest, and PGGM holds a 49% noncontrolling interest, in a joint venture (“ATC Europe”) comprised primarily of the Company’s operations in Germany and France, (iii) an approximate 75% controlling interest, and the South African investors hold an approximate 25% noncontrolling interest, in a subsidiary of the Company in South Africa and (iv) a 51% controlling interest in ATC Telecom Infrastructure Private Limited (“ATC TIPL”), formerly Viom Networks Limited (“Viom”), in India.
Significant Accounting Policies and Use of Estimates	Significant Accounting Policies and Use of Estimates —The preparation of financial statements in conformity with accounting principles generally accepted in the United States (“GAAP”) requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Actual results may differ from those estimates, and such differences could be material to the accompanying consolidated financial statements. The significant estimates in the accompanying consolidated financial statements include impairment of long-lived assets (including goodwill), asset retirement obligations, revenue recognition, rent expense, income taxes and accounting for business combinations and acquisitions of assets. The Company considers events or transactions that occur after the balance sheet date but before the financial statements are issued as additional evidence for certain estimates or to identify matters that require additional disclosure.
Accounts Receivable and Deferred Rent Asset	Accounts Receivable and Deferred Rent Asset —The Company derives the largest portion of its revenues, corresponding accounts receivable and the related deferred rent asset from a relatively small number of tenants in the telecommunications industry, and 53% of its current year revenues are derived from four tenants. The Company’s deferred rent asset is associated with non-cancellable tenant leases that contain fixed escalation clauses over the terms of the applicable lease in which revenue is recognized on a straight-line basis over the lease term. The Company mitigates its concentrations of credit risk with respect to notes and trade receivables and the related deferred rent assets by actively monitoring the creditworthiness of its borrowers and tenants. In recognizing tenant revenue, the Company assesses the collectibility of both the amounts billed and the portion recognized in advance of billing on a straight-line basis. This assessment takes tenant credit risk and business and industry conditions into consideration to ultimately determine the collectibility of the amounts billed. To the extent the amounts, based on management’s estimates, may not be collectible, recognition is deferred until such point as collectibility is determined to be reasonably assured. Any amounts that were previously recognized as revenue and subsequently determined to be uncollectible are charged to bad debt expense included in Selling, general, administrative and development expense in the accompanying consolidated statements of operations. Accounts receivable is reported net of allowances for doubtful accounts related to estimated losses resulting from a tenant’s inability to make required payments and allowances for amounts invoiced whose collectibility is not reasonably assured. These allowances are generally estimated based on payment patterns, days past due and collection history, and incorporate changes in economic conditions that may not be reflected in historical trends, such as tenants in bankruptcy, liquidation or reorganization. Receivables are written-off against the allowances when they are determined to be uncollectible. Such determination includes analysis and consideration of the particular conditions of the account.
Functional Currency	Functional Currency —The functional currency of each of the Company’s foreign operating subsidiaries is the respective local currency, except for Costa Rica, where the functional currency is the U.S. Dollar. All foreign currency assets and liabilities held by the subsidiaries are translated into U.S. Dollars at the exchange rate in effect at the end of the applicable fiscal reporting period and all foreign currency revenues and expenses are translated at the average monthly exchange rates. Translation adjustments are reflected in equity as a component of Accumulated Other Comprehensive Income Loss (“AOCL”) in the consolidated balance sheets and included as a component of Comprehensive income (loss) in the consolidated statements of comprehensive income (loss). Gains and losses on foreign currency transactions are reflected in Other expense in the consolidated statements of operations. However, the effect from fluctuations in foreign currency exchange rates on intercompany debt that for which repayment is not anticipated in the foreseeable future is reflected in AOCL in the consolidated balance sheets and included as a component of comprehensive income (loss).
Cash and Cash Equivalents	Cash and Cash Equivalents —Cash and cash equivalents include cash on hand, demand deposits and short-term investments with original maturities of three months or less. The Company maintains its deposits at high quality financial institutions and monitors the credit ratings of those institutions.
Restricted Cash	Restricted Cash— Restricted cash includes cash pledged as collateral to secure obligations and all cash whose use is otherwise limited by contractual provisions.
Short-Term Investments	Short-Term Investments— Short-term investments consists of highly liquid investments with original maturities in excess of three months.
Property and Equipment	Property and Equipment —Property and equipment is recorded at cost or, in the case of acquired properties at estimated fair value on the date acquired. Cost for self-constructed towers includes direct materials and labor, capitalized interest and certain indirect costs associated with construction of the tower, such as transportation costs, employee benefits and payroll taxes. The Company begins the capitalization of costs during the pre-construction period, which is the period during which costs are incurred to evaluate the site, and continues to capitalize costs until the tower is substantially completed and ready for occupancy by a tenant. Labor and related costs capitalized for the years ended December 31, 2017 , 2016 and 2015 were $50.9 million , $47.7 million and $44.7 million , respectively. Capitalized interest costs were not material for the years ended December 31, 2017 , 2016 and 2015 . Expenditures for repairs and maintenance are expensed as incurred. Augmentation and improvements that extend an asset’s useful life or enhance capacity are capitalized. Depreciation expense is recorded using the straight-line method over the assets’ estimated useful lives. Towers and related assets on leased land are depreciated over the shorter of the estimated useful life of the asset or the term of the corresponding ground lease, taking into consideration lease renewal options and residual value. Towers or assets acquired through capital leases are recorded net at the present value of future minimum lease payments or the fair value of the leased asset at the inception of the lease. Property and equipment and assets held under capital leases are amortized over the shorter of the applicable lease term or the estimated useful life of the respective assets for periods generally not exceeding twenty years. The Company reviews its tower portfolio for indicators of impairment on an individual tower basis. Impairments primarily result from a tower not having current tenant leases or from having expenses in excess of revenues. The Company reviews other long-lived assets for impairment whenever events, changes in circumstances or other evidence indicate that the carrying amount of the Company’s assets may not be recoverable. The Company records impairment charges in Other operating expenses in the consolidated statements of operations in the period in which the Company identifies such impairment.
Goodwill and Other Intangible Assets	Goodwill and Other Intangible Assets —The Company reviews goodwill for impairment at least annually (as of December 31) or whenever events or circumstances indicate the carrying value of an asset may not be recoverable. Goodwill is recorded in the applicable segment and assessed for impairment at the reporting unit level. The Company utilizes the two-step impairment test and employs a discounted cash flow analysis when testing goodwill for impairment. The key assumptions utilized in the discounted cash flow analysis include current operating performance, terminal sales growth rate, management’s expectations of future operating results and cash requirements, the current weighted average cost of capital and an expected tax rate. Under the first step of the test, the Company compares the fair value of the reporting unit, as calculated under an income approach using future discounted cash flows, to the carrying amount of the applicable reporting unit. If the carrying amount exceeds the fair value, the Company conducts the second step of this test, in which the implied fair value of the applicable reporting unit’s goodwill is compared to the carrying amount of that goodwill. If the carrying amount of goodwill exceeds its implied fair value, an impairment loss would be recognized for the amount of the excess. During the years ended December 31, 2017 , 2016 and 2015 , no potential impairment was identified under the first step of the test, as the fair value of each of the reporting units was in excess of its carrying amount. Intangible assets that are separable from goodwill and are deemed to have a definite life are amortized over their useful lives, generally ranging from three to twenty years and are evaluated separately for impairment at least annually or whenever events or circumstances indicate that the carrying amount of an asset may not be recoverable. The Company reviews its network location intangible assets for indicators of impairment on an individual tower basis. Impairments primarily result from a tower not having current tenant leases or from having expenses in excess of revenues. The Company monitors its tenant-related intangible assets on a tenant by tenant basis for indicators of impairment, such as high levels of turnover or attrition, non-renewal of a significant number of contracts or the cancellation or termination of a relationship. The Company assesses recoverability by determining whether the carrying amount of the related assets will be recovered primarily through projected undiscounted future cash flows. If the Company determines that the carrying amount of an asset may not be recoverable, the Company measures any impairment loss based on the projected future discounted cash flows to be provided from the asset or available market information relative to the asset’s fair value, as compared to the asset’s carrying amount. The Company records impairment charges in Other operating expenses in the consolidated statements of operations in the period in which the Company identifies such impairment.
Derivatives Financial Instruments	Derivative Financial Instruments —Derivatives are recorded on the consolidated balance sheet at fair value. If a derivative is designated as a cash flow hedge, the effective portions of changes in the fair value of the derivative are recorded in AOCL, as well as a component of comprehensive income (loss), and are recognized in the results of operations when the hedged item affects earnings. Changes in fair value of the ineffective portions of cash flow hedges are recognized in the results of operations. For derivative instruments that are designated and qualify as fair value hedges, changes in value of the derivatives are recorded in Other expense in the consolidated statements of operations in the current period, along with the offsetting gain or loss on the hedged item attributable to the hedged risk. For derivative instruments not designated as hedging instruments, changes in fair value are recognized in the results of operations in the period that the change occurs. The primary risks managed through the use of derivative instruments is interest rate risk, exposure to changes in the fair value of debt attributable to interest rate risk and currency risk. From time to time, the Company enters into interest rate swap agreements or foreign currency contracts to manage exposure to these risks. Under these agreements, the Company is exposed to counterparty credit risk to the extent that a counterparty fails to meet the terms of a contract. The Company’s exposure is limited to the current value of the contract at the time the counterparty fails to perform. The Company assesses, both at the inception of the hedge and on an ongoing basis, whether the derivatives that are used in hedging transactions are highly effective in offsetting changes in cash flows or fair values of hedged items. The Company does not hold derivatives for trading purposes.
Fair Value Measurements	Fair Value Measurements —The Company determines the fair value of its financial instruments based on the fair value hierarchy, which requires an entity to maximize the use of observable inputs and minimize the use of unobservable inputs when measuring fair value.
Asset Retirement Obligations	Asset Retirement Obligations —When required, the Company recognizes the fair value of obligations to remove its tower assets and remediate the leased land upon which certain of its tower assets are located. Generally, the associated retirement costs are capitalized as part of the carrying amount of the related tower assets and depreciated over their estimated useful lives and the liability is accreted through the obligation’s estimated settlement date. Fair value estimates of asset retirement obligations generally involve discounting of estimated future cash flows associated with takedown costs. Periodic accretion of such liabilities due to the passage of time is included in Depreciation, amortization and accretion expense in the consolidated statements of operations. Adjustments are also made to the asset retirement obligation liability to reflect changes in the estimates of timing and amount of expected cash flows, with an offsetting adjustment made to the related long-lived tangible asset. The significant assumptions used in estimating the Company’s aggregate asset retirement obligation are: timing of tower removals; cost of tower removals; timing and number of land lease renewals; expected inflation rates; and credit-adjusted, risk-free interest rates that approximate the Company’s incremental borrowing rate.
Income Taxes	Income Taxes —As a REIT, the Company generally is not subject to U.S. federal income taxes on income generated by its U.S. REIT operations. However, the Company remains obligated to pay U.S. federal income taxes on certain earnings and continues to be subject to taxation in its foreign jurisdictions. Accordingly, the consolidated financial statements reflect provisions for federal, state, local and foreign income taxes. The Company recognizes deferred tax assets and liabilities for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax basis, as well as operating loss and tax credit carryforwards. The Company measures deferred tax assets and liabilities using enacted tax rates expected to apply to taxable income in the years in which those temporary differences and carryforwards are expected to be recovered or settled. The effect on deferred tax assets and liabilities as a result of a change in tax rates is recognized in income in the period that includes the enactment date. The Company periodically reviews its deferred tax assets, and provides valuation allowances if, based on the available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. Management assesses the available positive and negative evidence to estimate if sufficient future taxable income will be generated to use the existing deferred tax assets. Valuation allowances would be reversed as a reduction to the provision for income taxes if related deferred tax assets are deemed realizable based on changes in facts and circumstances relevant to the assets’ recoverability. The Company classifies uncertain tax positions as non-current income tax liabilities unless expected to be paid within one year. The Company reports penalties and tax-related interest expense as a component of the income tax provision and interest income from tax refunds as a component of Other expense in the consolidated statements of operations.
Other Comprehensive Income (Loss)	Other Comprehensive Income (Loss) —Other comprehensive income (loss) refers to items excluded from net income that are recorded as an adjustment to equity, net of tax. The Company’s other comprehensive income (loss) primarily consisted of changes in fair value of effective derivative cash flow hedges, foreign currency translation adjustments and reclassification of unrealized losses on effective derivative cash flow hedges.
Distributions	Distributions —As a REIT, the Company must annually distribute to its stockholders an amount equal to at least 90% of its REIT taxable income (determined before the deduction for distributed earnings and excluding any net capital gain). Generally, the Company has distributed, and expects to continue to distribute, all or substantially all of its REIT taxable income after taking into consideration its utilization of net operating losses (“NOLs”). The amount, timing and frequency of future distributions will be at the sole discretion of the Board of Directors and will depend upon various factors, a number of which may be beyond the Company’s control, including the Company’s financial condition and operating cash flows, the amount required to maintain its qualification for taxation as a REIT and reduce any income and excise taxes that the Company otherwise would be required to pay, limitations on distributions in the Company’s existing and future debt and preferred equity instruments, the Company’s ability to utilize NOLs to offset the Company’s distribution requirements, limitations on its ability to fund distributions using cash generated through its TRSs and other factors that the Board of Directors may deem relevant.
Acquisitions	Acquisitions —For acquisitions that meet the definition of a business combination, the Company applies the acquisition method of accounting where assets acquired and liabilities assumed are recorded at fair value at the date of each acquisition, and the results of operations are included with those of the Company from the dates of the respective acquisitions. Any excess of the purchase price paid by the Company over the amounts recognized for assets acquired and liabilities assumed is recorded as goodwill. The Company continues to evaluate acquisitions for a period not to exceed one year after the applicable acquisition date of each transaction to determine whether any additional adjustments are needed to the allocation of the purchase price paid for the assets acquired and liabilities assumed. All other acquisitions are accounted for as asset acquisitions and the purchase price is allocated to the net assets acquired with no recognition of goodwill. The purchase price is not subsequently adjusted. The fair value of the assets acquired and liabilities assumed is typically determined by using either estimates of replacement costs or discounted cash flow valuation methods. When determining the fair value of tangible assets acquired, the Company must estimate the cost to replace the asset with a new asset taking into consideration such factors as age, condition and the economic useful life of the asset. When determining the fair value of intangible assets acquired and liabilities assumed, the Company must estimate the applicable discount rate and the timing and amount of future tenant cash flows, including rate and terms of renewal and attrition.
Revenue Recognition	Revenue Recognition —The Company’s revenue from leasing and similar arrangements, including fixed escalation clauses present in non-cancellable agreements, is reported on a straight-line basis over the term of the respective agreements when collectibility is reasonably assured. Escalation clauses tied to the Consumer Price Index (“CPI”) or other inflation-based indices, and other incentives present in agreements with the Company’s tenants are excluded from the straight-line calculation. Total property straight-line revenues for the years ended December 31, 2017 , 2016 and 2015 were $194.4 million , $131.7 million and $155.0 million , respectively. Amounts billed upfront in connection with the execution of lease and other agreements are initially deferred and reflected in Unearned revenue in the accompanying consolidated balance sheets and recognized as revenue over the terms of the applicable agreements. Amounts billed or received for services prior to being earned are deferred and reflected in Unearned revenue in the accompanying consolidated balance sheets until the criteria for recognition have been met. Services revenues are derived under contracts or arrangements with customers that provide for billings either on a fixed price basis or a variable price basis, which includes factors such as time and expenses. Revenues are recognized as or when services are performed, and may include estimates for percentage completed. Amounts billed or received for services prior to being earned are deferred and reflected in Unearned revenue in the accompanying consolidated balance sheets until the criteria for recognition have been met.
Rent Expense	Rent Expense —Many of the leases underlying the Company’s tower sites have fixed rent escalations, which provide for periodic increases in the amount of ground rent payable by the Company over time. In addition, certain of the Company’s tenant leases require the Company to exercise available renewal options pursuant to the underlying ground lease if the tenant exercises its renewal option. The Company calculates straight-line ground rent expense for these leases based on the fixed non-cancellable term of the underlying ground lease plus all periods, if any, for which failure to renew the lease imposes an economic penalty to the Company such that renewal appears to be reasonably assured. Total property straight-line ground rent expense for the years ended December 31, 2017 , 2016 and 2015 was $62.3 million , $67.8 million and $56.1 million , respectively. The Company records a liability for straight-line ground rent expense in Other non-current liabilities. The Company records prepaid ground rent in Prepaid and other current assets and Notes receivable and other non-current assets in the accompanying consolidated balance sheets according to the anticipated period of benefit.
Selling, General, Administrative and Development Expense	Selling, General, Administrative and Development Expense —Selling, general and administrative expense consists of overhead expenses related to the Company’s property and services operations and corporate overhead costs not specifically allocable to any of the Company’s individual business operations. Development expense consists of costs related to the Company’s acquisition efforts, costs associated with new business initiatives and project cancellation costs.
Stock-Based Compensation	Stock-Based Compensation —Stock-based compensation expense is measured at the accounting measurement date based on the fair value of the award and is generally recognized as an expense over the service period, which typically represents the vesting period. The Company provides for accelerated vesting and extended exercise periods of stock options and restricted stock units upon an employee’s death or permanent disability, or upon an employee’s qualified retirement, provided certain eligibility criteria are met. Accordingly, the Company recognizes compensation expense for stock options and time-based restricted stock units (“RSUs”) over the shorter of (i) the four -year vesting period or (ii) the period from the date of grant to the date the employee becomes eligible for such retirement benefits, which may occur upon grant. The expense recognized includes the impact of forfeitures as they occur. In March 2015, 2016 and 2017, the Company granted performance-based restricted stock units (“PSUs”) to its executive officers. Threshold, target and maximum parameters were established for the metrics for each year in the three -year performance period for the March 2015 grants, and for a three -year performance period for the March 2016 and 2017 grants. The metrics will be used to calculate the number of shares that will be issuable when the awards vest, which may range from zero to 200% of the target amounts. The Company recognizes compensation expense for PSUs over the three -year vesting period, subject to adjustment based on the date the employee becomes eligible for retirement benefits as well as performance relative to grant parameters. The fair value of stock options is determined using the Black-Scholes option-pricing model and the fair value of RSUs and PSUs is based on the fair value of the Company’s common stock on the date of grant. The Company recognizes all stock-based compensation expense in either Selling, general, administrative and development expense, costs of operations or as part of the costs associated with the construction of the tower assets.
Litigation Costs	Litigation Costs —The Company periodically becomes involved in various claims and lawsuits that are incidental to its business. The Company regularly monitors the status of pending legal actions to evaluate both the magnitude and likelihood of any potential loss. The Company accrues for these potential losses when it is probable that a liability has been incurred and the amount of loss, or possible range of loss, can be reasonably estimated. Should the ultimate losses on contingencies or litigation vary from estimates, adjustments to those liabilities may be required. The Company also incurs legal costs in connection with these matters and records estimates of these expenses, which are reflected in Selling, general, administrative and development expense in the accompanying consolidated statements of operations.
Earnings Per Common Share-Basic and Diluted	Earnings Per Common Share — Basic and Diluted —Basic net income per common share represents net income attributable to American Tower Corporation common stockholders divided by the weighted average number of common shares outstanding during the period. Diluted net income per common share represents net income attributable to American Tower Corporation common stockholders divided by the weighted average number of common shares outstanding during the period and any dilutive common share equivalents, including (A) shares issuable upon (i) the vesting of RSUs, (ii) exercise of stock options, and (iii) conversion of the Company’s mandatory convertible preferred stock and (B) shares earned upon the achievement of the parameters established for the PSUs, each to the extent not anti-dilutive. Dilutive common share equivalents also include the dilutive impact of the shares issuable in the Alltel transaction, which is described in notes 15 and 18. The Company uses the treasury stock method to calculate the effect of its outstanding RSUs, PSUs and stock options and uses the if-converted method to calculate the effect of its outstanding mandatory convertible preferred stock.
Retirement Plan	Retirement Plan —The Company has a 401(k) plan covering substantially all employees who meet certain age and employment requirements.
Accounting Standards Updates	Accounting Standards Updates —In May 2014, the Financial Accounting Standards Board (the “FASB”) issued new guidance on revenue recognition, which requires an entity to recognize revenue in an amount that reflects the consideration to which the entity expects to be entitled in exchange for the transfer of promised goods or services to customers. The standard will replace most existing revenue recognition guidance and will become effective for the Company on January 1, 2018. The standard permits the use of either the retrospective or cumulative effect transition method. Leases are not included in the scope of this standard. The revenue to which the Company must apply this standard is generally limited to services revenue, certain power and fuel charges and other fees charged to tenants. As of December 31, 2017, this revenue was approximately 14% of total revenue. The Company is finalizing the required disclosures and has completed its analysis of the impact of this standard and has determined that the impact on the timing of revenue recognition as a result of its adoption will not have a material effect on the Company’s financial statements. The Company intends to adopt this standard using a modified retrospective approach. In January 2016, the FASB issued new guidance on the recognition and measurement of financial assets and financial liabilities. The guidance amends certain aspects of recognition, measurement, presentation and disclosure of financial instruments. This standard is effective for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2017. The Company does not expect the adoption of this guidance to have a material effect on its financial statements. In February 2016, the FASB issued new guidance on the accounting for leases. The guidance amends the existing accounting standards for lease accounting, including the requirement that lessees recognize right of use assets and lease liabilities for leases with terms greater than twelve months in the statement of financial position. Under the new guidance, lessor accounting is largely unchanged. This guidance is effective for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2018. The standard is required to be applied using a modified retrospective approach for all leases existing at, or entered into after, the beginning of the earliest comparative period presented. The Company (i) has established a multidisciplinary team to assess and implement the new guidance, (ii) expects the guidance to have a material impact on its consolidated balance sheets due to the recording of right of use assets and lease liabilities for leases in which it is a lessee and which it currently treats as operating leases and (iii) continues to evaluate the impact of the new guidance. In November 2016, the FASB issued new guidance on amounts described as restricted cash or restricted cash equivalents within the statement of cash flows. The guidance requires amounts generally described as restricted cash and restricted cash equivalents be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period balances on the statement of cash flows. The guidance is effective for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2017. The standard is required to be applied using a retrospective transition method to each period presented. The Company early adopted this guidance during the fourth quarter of 2017. The adoption of this guidance did not have a material effect on the Company’s financial statements. In January 2017, the FASB issued new guidance that clarifies the definition of a business that an entity uses to determine whether a transaction should be accounted for as an asset acquisition (or disposal) or a business combination. The Company early adopted this guidance during the first quarter of 2017. As a result, more transactions have been accounted for as asset acquisitions instead of business combinations. In January 2017, the FASB issued new guidance on accounting for goodwill impairments. The guidance eliminates Step 2 from the goodwill impairment test and requires, among other things, recognition of an impairment loss when the carrying value of a reporting unit exceeds its fair value. The loss recognized is limited to the total amount of goodwill allocated to that reporting unit. The guidance is effective for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. The Company does not expect the adoption of this guidance to have a material effect on the Company’s financial statements. In May 2017, the FASB issued new guidance on accounting for stock-based compensation. The guidance clarifies when changes to the terms or conditions of a share-based payment award must be accounted for as modifications. The guidance is effective for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2017. The Company early adopted this guidance during the second quarter of 2017. The adoption of this guidance did not have a material effect on the Company’s financial statements. In August 2017, the FASB issued new guidance on hedge and derivative accounting. The guidance simplifies accounting rules around hedge accounting and the disclosures of hedging arrangements. Among other things, the guidance eliminates the need to separately measure and report hedge ineffectiveness and generally requires the entire change in fair value of a hedging instrument to be presented in the same income statement line as the hedged item. The guidance is effective for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2018, with early adoption permitted. The Company does not expect the adoption of this guidance to have a material effect on the Company’s financial statements. In January 2018, the FASB issued new guidance on the treatment of land easements. The guidance provides a practical expedient to not evaluate existing or expired land easements under the new lease accounting standards if those easements were not previously accounted for as leases under the existing lease guidance. The Company does not expect the adoption of this guidance to have a material effect on the Company’s financial statements or its adoption of the lease accounting guidance. In February 2018, the FASB issued new guidance on the treatment of tax effects that are presented in other comprehensive income. The guidance allows a reclassification from accumulated other comprehensive income to retained earnings for stranded tax effects as a result of the December 2017 legislation commonly referred to as the Tax Cuts and Jobs Act (“the Tax Act”). The guidance is effective for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2018, with early adoption permitted. The Company does not expect the adoption of this guidance to have a material effect on the Company’s financial statements.

BUSINESS AND SUMMARY OF SIGNI35

BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)	12 Months Ended
	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Changes in allowances	Changes in the allowances were as follows: Year Ended December 31, 2017 2016 2015 Balance as of January 1, $ 45.9 $ 23.1 $ 17.3 Current year increases 87.2 50.0 19.9 Write-offs, recoveries and other (1) (2.1 ) (27.2 ) (14.1 ) Balance as of December 31, $ 131.0 $ 45.9 $ 23.1 _______________ (1) Recoveries includes recognition of revenue resulting from collections of previously reserved amounts.
Schedule of cash, cash equivalents, and restricted cash	The reconciliation of cash and cash equivalents, and restricted cash reported within the statement of financial position that sum to the total of the same such amounts shown in the statement of cash flows is as follows: Year Ended December 31, 2017 2016 2015 Cash and cash equivalents $ 802.1 $ 787.2 $ 320.7 Restricted cash 152.8 149.3 142.2 Total cash, cash equivalents and restricted cash $ 954.9 $ 936.5 $ 462.9

PREPAID AND OTHER CURRENT ASS36

PREPAID AND OTHER CURRENT ASSETS (Tables)	12 Months Ended
	Dec. 31, 2017
Prepaid Expense and Other Assets, Current [Abstract]
Prepaid and other current assets	Prepaid and other current assets consisted of the following as of December 31,: 2017 2016 Prepaid operating ground leases $ 148.6 134.2 Prepaid income tax 136.5 127.1 Unbilled receivables 107.9 57.7 Value added tax and other consumption tax receivables 64.2 31.6 Prepaid assets 39.6 36.3 Other miscellaneous current assets 71.8 54.1 Prepaids and other current assets $ 568.6 $ 441.0

PROPERTY AND EQUIPMENT (Tables)

PROPERTY AND EQUIPMENT (Tables)	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Property and equipment	Property and equipment (including assets held under capital leases) consisted of the following as of December 31,: Estimated Useful Lives (years) (1) 2017 2016 Towers Up to 20 $ 12,500.5 $ 11,740.5 Equipment 2 - 15 1,423.0 1,176.3 Buildings and improvements 3 - 32 631.4 621.9 Land and improvements (2) Up to 20 2,112.9 1,909.7 Construction-in-progress 282.1 203.4 Total 16,949.9 15,651.8 Less accumulated depreciation (5,848.9 ) (5,134.5 ) Property and equipment, net $ 11,101.0 $ 10,517.3 _______________ (1) Assets on leased land are depreciated over the shorter of the estimated useful life of the asset or the term of the corresponding ground lease taking into consideration lease renewal options and residual value. (2) Estimated useful lives apply to improvements only.

GOODWILL AND OTHER INTANGIBLE38

GOODWILL AND OTHER INTANGIBLE ASSETS (Tables)	12 Months Ended
	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Changes in the carrying value of goodwill	The changes in the carrying value of goodwill for the Company’s business segments were as follows: Property Services Total U.S. Asia EMEA Latin America Balance as of January 1, 2016 $ 3,379.2 $ 170.7 $ 132.6 $ 407.4 $ 2.0 $ 4,091.9 Additions — 881.8 (1) 40.4 53.5 — 975.7 Effect of foreign currency translation — (23.2 ) (22.5 ) 48.8 — 3.1 Balance as of January 1, 2017 $ 3,379.2 $ 1,029.3 $ 150.5 $ 509.7 $ 2.0 $ 5,070.7 Additions (2) — 0.4 220.9 264.8 — 486.1 Effect of foreign currency translation — 65.3 33.5 (17.2 ) — 81.6 Balance as of December 31, 2017 $ 3,379.2 $ 1,095.0 $ 404.9 $ 757.3 $ 2.0 $ 5,638.4 _______________ (1) Assumed in the acquisition of Viom (see note 6). (2) Additions consist of $485.1 million resulting from 2017 acquisitions and $1.0 million from revisions to prior year acquisitions resulting from measurement period adjustments.
Finite Intangible Assets	The Company’s other intangible assets subject to amortization consisted of the following: As of December 31, 2017 As of December 31, 2016 Estimated Useful Lives Gross Carrying Value Accumulated Amortization Net Book Value Gross Carrying Value Accumulated Amortization Net Book Value (years) Acquired network location intangibles (1) Up to 20 $ 4,858.8 $ (1,525.3 ) $ 3,333.5 $ 4,622.3 $ (1,280.3 ) $ 3,342.0 Acquired tenant-related intangibles 15-20 11,150.9 (2,754.7 ) 8,396.2 10,130.5 (2,224.1 ) 7,906.4 Acquired licenses and other intangibles 3-20 58.8 (8.1 ) 50.7 28.1 (4.8 ) 23.3 Economic Rights, TV Azteca 70 14.5 (11.6 ) 2.9 13.9 (11.0 ) 2.9 Total other intangible assets $ 16,083.0 $ (4,299.7 ) $ 11,783.3 $ 14,794.8 $ (3,520.2 ) $ 11,274.6 _______________ (1) Acquired network location intangibles are amortized over the shorter of the term of the corresponding ground lease taking into consideration lease renewal options and residual value or up to 20 years, as the Company considers these intangibles to be directly related to the tower assets.
Expected future amortization expenses	Based on current exchange rates, the Company expects to record amortization expense as follows over the next five years: Year Ending December 31, 2018 $ 810.5 2019 806.7 2020 787.2 2021 768.7 2022 766.1

NOTES RECEIVABLE AND OTHER NO39

NOTES RECEIVABLE AND OTHER NON-CURRENT ASSETS (Tables)	12 Months Ended
	Dec. 31, 2017
Notes Receivable And Other Long Term Assets [Abstract]
Notes receivable and other non-current assets	Notes receivable and other non-current assets consisted of the following as of December 31,: 2017 2016 Long-term prepaid ground rent $ 552.8 $ 467.8 Notes receivable 83.7 83.7 Other miscellaneous assets 313.6 290.0 Notes receivable and other non-current assets $ 950.1 $ 841.5

ACQUISITIONS (Tables)

ACQUISITIONS (Tables)	12 Months Ended
	Dec. 31, 2017
Business Combinations [Abstract]
Schedule of merger and acquisition related costs	During the years ended December 31, 2017 , 2016 and 2015 , the Company recorded the following acquisition and merger related expenses for business combinations and integration costs: Year Ended December 31, 2017 2016 2015 Acquisition and merger related expenses $ 16.3 $ 15.9 $ 18.8 Integration costs $ 11.5 $ 9.9 $ 18.1
Schedule of recognized identified assets acquired and liabilities assumed	The following table summarizes the allocations of the purchase prices for the fiscal year 2017 acquisitions based upon their estimated fair value at the date of acquisition: EMEA Latin America FPS Towers France (1) Mexico (1) Other (2) (3) Final Allocation Preliminary Allocation Current assets $ 34.5 $ 44.4 $ 12.7 Non-current assets 15.0 — 19.7 Property and equipment 122.9 94.0 290.0 Intangible assets (4): Tenant-related intangible assets 440.7 153.3 364.7 Network location intangible assets 113.0 — 154.3 Other intangible assets 8.5 22.0 — Current liabilities (29.0 ) (28.8 ) (10.5 ) Deferred tax liability (135.4 ) (38.8 ) (2.7 ) Other non-current liabilities (19.9 ) (4.5 ) (14.2 ) Net assets acquired 550.3 241.6 814.0 Goodwill (5) 220.9 264.2 — Fair value of net assets acquired 771.2 505.8 814.0 Debt assumed — — — Purchase price $ 771.2 $ 505.8 $ 814.0 _______________ (1) Accounted for as a business combination. (2) Accounted for as asset acquisitions. (3) Includes 127 sites in Peru held pursuant to long-term capital leases. (4) Tenant-related intangible assets, network location intangible assets and other intangible assets are amortized on a straight-line basis over periods of up to 20 years. (5) Primarily results from purchase accounting adjustments, which are not deductible for tax purposes. The following table summarizes the preliminary and updated allocations of the purchase prices paid and the amounts of assets acquired and liabilities assumed for the fiscal year 2016 acquisitions based upon their estimated fair value at the date of acquisition. Balances are reflected in the accompanying consolidated balance sheet as of December 31, 2017. Preliminary Allocation (1) Updated Allocation Asia Other (2) Asia Other (2) Viom Viom (3) Current assets $ 276.6 $ 25.5 $ 281.9 $ 24.5 Non-current assets 57.6 2.3 52.3 2.3 Property and equipment 702.0 81.5 705.8 81.5 Intangible assets (4): Tenant-related intangible assets 1,369.6 105.6 1,369.6 105.6 Network location intangible assets 666.4 83.6 666.4 83.6 Current liabilities (195.9 ) (14.8 ) (201.1 ) (14.8 ) Deferred tax liability (619.1 ) (43.8 ) (619.1 ) (43.4 ) Other non-current liabilities (102.8 ) (29.4 ) (101.8 ) (29.4 ) Net assets acquired 2,154.4 210.5 2,154.0 209.9 Goodwill (5) 881.8 93.9 882.2 94.5 Fair value of net assets acquired 3,036.2 304.4 3,036.2 304.4 Debt assumed (786.8 ) — (786.8 ) — Redeemable noncontrolling interests (1,100.9 ) — (1,100.9 ) — Purchase Price $ 1,148.5 $ 304.4 $ 1,148.5 $ 304.4 _______________ (1) As reported for the year ended December 31, 2016. (2) Of the total purchase price, $12.1 million was reflected in Accounts payable in the consolidated balance sheet as of December 31, 2016. (3) The allocation of the purchase price for the Viom Acquisition was finalized during the year ended December 31, 2017. (4) Tenant-related intangible assets and network location intangible assets are amortized on a straight-line basis over periods of up to 20 years. (5) Primarily results from purchase accounting adjustments, which are at least partially deductible for tax purposes.
Schedule of pro forma information	The following table presents the unaudited pro forma financial results as if the 2017 acquisitions had occurred on January 1, 2016 and the 2016 acquisitions had occurred on January 1, 2015 . The pro forma results do not include any anticipated cost synergies, costs or other integration impacts. Accordingly, such pro forma amounts are not necessarily indicative of the results that actually would have occurred had the transactions been completed on the dates indicated, nor are they indicative of the future operating results of the Company. Year Ended December 31, 2017 2016 Pro forma revenues $ 6,775.3 $ 6,240.6 Pro forma net income attributable to American Tower Corporation common stockholders $ 1,145.5 $ 822.8 Pro forma net income per common share amounts: Basic net income attributable to American Tower Corporation common stockholders $ 2.68 $ 1.94 Diluted net income attributable to American Tower Corporation common stockholders $ 2.65 $ 1.92
Schedule of contingent consideration changes	A summary of the value of the Company’s contingent consideration obligations are as follows: Year Ended December 31, 2017 Maximum potential value (1) Estimated value at December 31, 2017 Additions Settlements Change in Fair Value Colombia $ — $ — $ — $ — $ (5.4 ) Ghana 0.6 0.6 — — 0.0 South Africa 9.1 9.1 — — (0.9 ) United States 0.4 0.4 — — 0.0 Total $ 10.1 $ 10.1 $ — $ — $ (6.3 ) _______________ (1) The maximum potential value is based on exchange rates at December 31, 2017 . The minimum value would be no less than $9.1 million . The changes in fair value of the contingent consideration were as follows during the years ended December 31,: 2017 2016 Balance as of January 1 $ 15.4 $ 12.4 Additions — 8.8 Settlements — (0.3 ) Change in fair value (6.3 ) (6.4 ) Foreign currency translation adjustment 1.0 0.9 Balance as of December 31 $ 10.1 $ 15.4

ACCRUED EXPENSES (Tables)

ACCRUED EXPENSES (Tables)	12 Months Ended
	Dec. 31, 2017
Accrued Liabilities, Current [Abstract]
Schedule of accrued expenses	Accrued expenses consisted of the following as of December 31,: 2017 2016 Accrued property and real estate taxes $ 154.4 $ 138.4 Payroll and related withholdings 82.2 76.1 Amounts payable to tenants 60.8 32.3 Accrued rent 54.0 51.0 Accrued income tax payable 15.3 11.6 Accrued pass-through costs 59.7 68.5 Accrued construction costs 31.9 28.6 Accrued pass-through taxes 25.3 1.0 Other accrued expenses 370.7 213.0 Accrued expenses $ 854.3 $ 620.5

LONG-TERM OBLIGATIONS (Tables)

LONG-TERM OBLIGATIONS (Tables)	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Schedule of Long-term Debt Instruments	Amounts outstanding and key terms of other subsidiary debt consisted of the following as of December 31, (in millions, except percentages): Carrying Value (Functional Currency) Carrying Value (USD) (1) Interest Rate Maturity Date 2017 2016 2017 2016 BR Towers Debentures (2) 306.8 329.3 $ 92.7 $ 101.0 7.400 % October 15, 2023 South African Credit Facility (3) 866.0 1,157.6 $ 69.9 $ 84.3 9.108 % December 17, 2020 Colombian Credit Facility (4) 138,740.3 168,286.5 $ 46.5 $ 56.1 8.513 % April 24, 2021 Brazil Credit Facility (5) 122.4 145.4 $ 37.0 $ 44.6 Various January 15, 2022 _______________ (1) Includes applicable deferred financing costs. (2) Denominated in BRL, with an original principal amount of 300.0 million BRL. Debt accrues interest at a variable rate. The aggregate principal amount of the BR Towers Debentures may be adjusted periodically relative to changes in the National Extended Consumer Price Index. (3) Denominated in ZAR, with an original principal amount of 830.0 million ZAR. On December 23, 2016, the borrower borrowed an additional 500.0 million ZAR. Debt accrues interest at a variable rate. (4) Denominated in COP, with an original principal amount of 200.0 billion COP. Debt accrues interest at a variable rate. The loan agreement for the Colombian Credit Facility requires that the borrower manage exposure to variability in interest rates on certain of the amounts outstanding under the Colombian Credit Facility. (5) Denominated in BRL, with an original principal amount of 271.0 million BRL. Debt accrues interest at a variable rate. As of December 30, 2016, the borrower no longer maintains the ability to draw on the Brazil Credit Facility. Outstanding amounts under the Company’s long-term obligations, reflecting discounts, premiums, debt issuance costs and fair value adjustments due to interest rate swaps consisted of the following as of December 31,: 2017 2016 Contractual Interest Rate (1) Maturity Date (1) 2013 Credit Facility (2) $ 2,075.6 $ 540.0 2.649 % June 28, 2021 Term Loan (2) 994.5 993.9 2.790 % January 31, 2023 2014 Credit Facility (2) 495.0 1,385.0 2.820 % January 31, 2023 4.500% senior notes — 998.7 N/A N/A 3.40% senior notes 999.8 999.7 3.400 % February 15, 2019 7.25% senior notes — 297.0 N/A N/A 2.800% senior notes 746.3 744.9 2.800 % June 1, 2020 5.050% senior notes 698.0 697.4 5.050 % September 1, 2020 3.300% senior notes 746.0 744.8 3.300 % February 15, 2021 3.450% senior notes 645.1 643.8 3.450 % September 15, 2021 5.900% senior notes 497.8 497.3 5.900 % November 1, 2021 2.250% senior notes 572.4 572.8 2.250 % January 15, 2022 4.70% senior notes 696.7 696.0 4.700 % March 15, 2022 3.50% senior notes 990.9 989.3 3.500 % January 31, 2023 3.000% senior notes 692.5 — 3.000 % June 15, 2023 5.00% senior notes 1,002.4 1,002.7 5.000 % February 15, 2024 1.375% senior notes 589.1 — 1.375 % April 4, 2025 4.000% senior notes 741.0 740.0 4.000 % June 1, 2025 4.400% senior notes 495.6 495.2 4.400 % February 15, 2026 3.375% senior notes 984.8 983.4 3.375 % October 15, 2026 3.125% senior notes 397.1 396.7 3.125 % January 15, 2027 3.55% senior notes 742.8 — 3.550 % July 15, 2027 3.600% senior notes 691.1 — 3.600 % January 15, 2028 Total American Tower Corporation debt 16,494.5 14,418.6 Series 2013-1A Securities (3) 499.8 498.6 1.551 % March 15, 2018 Series 2013-2A Securities (4) 1,291.8 1,290.3 3.070 % March 15, 2023 Series 2015-1 Notes (5) 348.0 347.1 2.350 % June 15, 2020 Series 2015-2 Notes (6) 520.1 519.4 3.482 % June 16, 2025 2012 GTP Notes — 179.5 N/A N/A Unison Notes — 133.0 N/A N/A India indebtedness (7) 512.6 549.5 7.90% - 9.55% Various India preference shares (8) 26.1 24.5 10.250 % March 2, 2020 Shareholder loans (9) 100.6 151.1 Various Various Other subsidiary debt (1) (10) 246.1 286.0 Various Various Total American Tower subsidiary debt 3,545.1 3,979.0 Other debt, including capital lease obligations 165.5 135.9 Total 20,205.1 18,533.5 Less current portion long-term obligations (774.8 ) (238.8 ) Long-term obligations $ 19,430.3 $ 18,294.7 _______________ (1) Represents the interest rate or maturity date as of December 31, 2017; interest rate does not reflect the impact of interest rate swap agreements. (2) Accrues interest at a variable rate. (3) Maturity date reflects the anticipated repayment date; final legal maturity is March 15, 2043. (4) Maturity date reflects the anticipated repayment date; final legal maturity is March 15, 2048. (5) Maturity date reflects the anticipated repayment date; final legal maturity is June 15, 2045. (6) Maturity date reflects the anticipated repayment date; final legal maturity is June 15, 2050. (7) Denominated in INR. Includes India working capital facility, remaining debt assumed by the Company in connection with the Viom Acquisition and debt that has been entered into by ATC TIPL. (8) Mandatorily redeemable preference shares (the “Preference Shares”) classified as debt. The Preference Shares are to be redeemed on March 2, 2020 and have a dividend rate of 10.25% per annum. (9) Reflects balances owed to the Company’s joint venture partners in Ghana and Uganda. The Ghana loan is denominated in Ghanaian Cedi (“GHS”) and the Uganda loan is denominated in Ugandan Shillings (“UGX”). (10) Includes the BR Towers debentures and the Brazil Credit Facility (as defined below), which are denominated in Brazilian Reais (“BRL”) and amortize through October 15, 2023 and January 15, 2022, respectively, the South African Credit Facility (as defined below), which is denominated in South African Rand (“ZAR”) and amortizes through December 17, 2020 and the Colombian Credit Facility (as defined below), which is denominated in Colombian Pesos (“COP”) and amortizes through April 24, 2021
Schedule of Line of Credit Facilities	As of December 31, 2017 , the key terms under the 2013 Credit Facility, the 2014 Credit Facility and the Term Loan were as follows: Outstanding Principal Balance Undrawn letters of credit Maturity Date Current margin over LIBOR Current commitment fee (1) 2013 Credit Facility $ 2,075.6 (2) $ 4.0 June 28, 2021 (3) 1.125 % 0.125 % 2014 Credit Facility $ 495.0 (2) $ 6.3 January 31, 2023 (3) 1.250 % 0.150 % Term Loan $ 1,000.0 (2) $ — January 31, 2023 1.250 % N/A _______________ (1) Fee on undrawn portion of each credit facility. (2) Borrowed at LIBOR. (3) Subject to two optional renewal periods.
Schedule of Debt Discounts	The following table outlines key terms related to the Company ’ s outstanding senior notes as of December 31, 2017 : Adjustments to Principal Amount (1) Aggregate Principal Amount 2017 2016 Interest payments due (2) Issue Date Par Call Date (3) 3.40% Notes (4) 1,000.0 (0.2 ) (0.3 ) February 15 and August 15 August 19, 2013 N/A 2.800% Notes 750.0 (3.7 ) (5.1 ) June 1 and December 1 May 7, 2015 May 1, 2020 5.050% Notes 700.0 (2.0 ) (2.6 ) March 1 and September 1 August 16, 2010 N/A 3.300% Notes 750.0 (4.0 ) (5.2 ) February 15 and August 15 January 12, 2016 January 15, 2021 3.450% Notes 650.0 (4.9 ) (6.2 ) March 15 and September 15 August 7, 2014 N/A 5.900% Notes 500.0 (2.2 ) (2.7 ) May 1 and November 1 October 6, 2011 N/A 2.250% Notes (5) 600.0 (27.6 ) (27.2 ) January 15 and July 15 September 30, 2016 N/A 4.70% Notes 700.0 (3.3 ) (4.0 ) March 15 and September 15 March 12, 2012 N/A 3.50% Notes 1,000.0 (9.1 ) (10.7 ) January 31 and July 31 January 8, 2013 N/A 3.000% Notes (6) 700.0 (7.5 ) — June 15 and December 15 December 8, 2017 N/A 5.00% Notes (4) 1,000.0 2.4 2.7 February 15 and August 15 August 19, 2013 N/A 1.375% Notes (7) 600.2 (11.1 ) — April 4 April 6, 2017 January 4, 2025 4.000% Notes 750.0 (9.0 ) (10.0 ) June 1 and December 1 May 7, 2015 March 1, 2025 4.400% Notes 500.0 (4.4 ) (4.8 ) February 15 and August 15 January 12, 2016 November 15, 2025 3.375% Notes 1,000.0 (15.2 ) (16.6 ) April 15 and October 15 May 13, 2016 July 15, 2026 3.125% Notes 400.0 (2.9 ) (3.3 ) January 15 and July 15 September 30, 2016 October 15, 2026 3.55% Notes 750.0 (7.2 ) — January 15 and July 15 June 30, 2017 April 15, 2027 3.600% Notes 700.0 (8.9 ) — January 15 and July 15 December 8, 2017 October 15, 2027 _______________ (1) Includes unamortized discounts, premiums and debt issuance costs and fair value adjustments due to interest rate swaps. (2) Interest payments are due semi-annually for each series of senior notes, except for the 1.375% Notes, for which interest payments are due annually on April 4. (3) The Company will not be required to pay a make-whole premium if redeemed on or after the par call date. (4) The original issue date for the 3.40% Notes and the 5.00% Notes was August 19, 2013. The issue date for the reopened 3.40% Notes and the reopened 5.00% Notes was January 10, 2014. (5) Includes $23.7 million and $22.3 million fair value adjustment due to interest rate swaps in 2017 and 2016, respectively. (6) Includes $0.8 million fair value adjustment due to interest rate swaps. (7) Note is denominated in Euro.
Schedule of India Indebtedness	Amounts outstanding and key terms of the India indebtedness consisted of the following as of December 31, 2017 (in millions, except percentages): Amount Outstanding (INR) Amount Outstanding (USD) Interest Rate (Range) Maturity Date (Range) Term loans 26,740 $ 418.7 7.90% - 8.65% January 24, 2018 - November 30, 2024 Debenture 6,000 $ 93.9 9.55 % April 28, 2020 Working capital facilities 0 $ 0 8.05% - 8.75% March 18, 2018 - October 23, 2018
Schedule of Shareholder Loans	The portions of the loans made by the Company’s wholly owned subsidiaries are eliminated in consolidation and the portions of the loans made by each of the Company’s joint venture partner’s wholly owned subsidiaries are reported as outstanding debt of the Company. Outstanding amounts under each of the Company’s shareholder loans consisted of the following as of December 31,: 2017 2016 Contractual Interest Rate Maturity Date Ghana loan (1) $ 66.5 $ 71.0 21.87 % December 31, 2019 Uganda loan (2) 34.1 80.1 16.80 % December 31, 2023 _______________ (1) Denominated in GHS. As of December 31, 2017, the aggregate principal amount outstanding under the Ghana loan was 300.9 million GHS. (2) Denominated in UGX. Effective January 1, 2017, the Uganda loan, which had an outstanding balance of $80.0 million and accrued interest at a variable rate, was converted by the holder to a new shareholder note for 114.5 billion UGX ( $31.8 million at the time of conversion), bearing interest at a fixed rate of 16.8% per annum. The remaining balance of the Uganda loan was converted into equity. As of December 31, 2017, the aggregate principal amount outstanding under the Uganda loan was 124.1 billion UGX.
Schedule of Maturities of Long-term Debt	Aggregate principal maturities of long-term debt, including capital leases, for the next five years and thereafter are expected to be: Year Ending December 31, 2018 $ 775.0 2019 1,192.6 2020 2,034.0 2021 4,051.2 2022 1,388.7 Thereafter 10,908.1 Total cash obligations 20,349.6 Unamortized discounts, premiums and debt issuance costs and fair value adjustments, net (144.5 ) Balance as of December 31, 2017 $ 20,205.1

ASSET RETIREMENT OBLIGATIONS (T

ASSET RETIREMENT OBLIGATIONS (Tables)	12 Months Ended
	Dec. 31, 2017
Asset Retirement Obligation [Abstract]
Carrying value of asset retirement obligations	The changes in the carrying amount of the Company’s asset retirement obligations were as follows: 2017 2016 Beginning balance as of January 1, $ 965.5 $ 856.9 Additions 33.4 64.1 Accretion expense 94.5 67.0 Revisions in estimates (1) 86.6 (21.1 ) Settlements (4.7 ) (1.4 ) Balance as of December 31, $ 1,175.3 $ 965.5 _______________ (1) Revisions in estimates include an increase to the liability of $13.0 million and $9.6 million related to foreign currency translation for the years ended December 31, 2017 and 2016, respectively

FAIR VALUE MEASUREMENTS (Tables

FAIR VALUE MEASUREMENTS (Tables)	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Schedule of fair value	The fair value of the Company’s financial assets and liabilities that are required to be measured on a recurring basis at fair value was as follows: December 31, 2017 December 31, 2016 Fair Value Measurements Using Fair Value Measurements Using Level 1 Level 2 Level 3 Level 1 Level 2 Level 3 Assets: Short-term investments (1) $ 1.0 — — $ 4.0 — — Interest rate swap agreements — — — — $ 0.0 — Embedded derivative in lease agreement — — $ 12.4 — — $ 13.3 Liabilities: Interest rate swap agreements — $ 29.0 — — $ 24.7 — Acquisition-related contingent consideration — — $ 10.1 — — $ 15.4 Fair value of debt related to interest rate swap agreements $ (24.5 ) — — $ (22.3 ) — — _______________ (1) Consists of highly liquid investments with original maturities in excess of three months.
Schedule of contingent consideration changes	A summary of the value of the Company’s contingent consideration obligations are as follows: Year Ended December 31, 2017 Maximum potential value (1) Estimated value at December 31, 2017 Additions Settlements Change in Fair Value Colombia $ — $ — $ — $ — $ (5.4 ) Ghana 0.6 0.6 — — 0.0 South Africa 9.1 9.1 — — (0.9 ) United States 0.4 0.4 — — 0.0 Total $ 10.1 $ 10.1 $ — $ — $ (6.3 ) _______________ (1) The maximum potential value is based on exchange rates at December 31, 2017 . The minimum value would be no less than $9.1 million . The changes in fair value of the contingent consideration were as follows during the years ended December 31,: 2017 2016 Balance as of January 1 $ 15.4 $ 12.4 Additions — 8.8 Settlements — (0.3 ) Change in fair value (6.3 ) (6.4 ) Foreign currency translation adjustment 1.0 0.9 Balance as of December 31 $ 10.1 $ 15.4

INCOME TAXES (Tables)

INCOME TAXES (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income tax provision from continuing operations	The income tax provision from continuing operations consisted of the following for the years ended December 31,: 2017 2016 2015 Current: Federal $ (0.1 ) $ (26.5 ) $ (73.9 ) State (3.8 ) (2.0 ) (21.2 ) Foreign (113.4 ) (100.1 ) (55.1 ) Deferred: Federal 0.2 (0.6 ) 9.1 State 1.0 (0.3 ) — Foreign 85.4 (26.0 ) (16.9 ) Income tax provision $ (30.7 ) $ (155.5 ) $ (158.0 )
Reconciliation between the U.S. statutory rate and the effective rate from continuing operations	Reconciliation between the U.S. statutory rate and the effective rate from continuing operations is as follows for the years ended December 31: 2017 2016 2015 Statutory tax rate 35 % 35 % 35 % Adjustment to reflect REIT status (1) (35 ) (35 ) (35 ) Foreign taxes 1 5 3 Foreign withholding taxes 3 4 3 Uncertain tax positions — 5 — Changes in tax laws (2 ) — 2 MIPT tax election (2) — — 11 Effective tax rate 2 % 14 % 19 % _______________ (1) As a result of the ability to utilize the dividends paid deduction to offset our REIT income and gains. (2) Includes federal and state taxes, net of federal benefit.
Components of income from continuing operations before income taxes and income on equity method investments	The domestic and foreign components of income from continuing operations before income taxes are as follows for the years ended December 31,: 2017 2016 2015 United States $ 971.2 $ 882.6 $ 785.2 Foreign 284.9 243.3 44.8 Total $ 1,256.1 $ 1,125.9 $ 830.0
Components of the net deferred tax asset and related valuation allowance	The components of the net deferred tax asset and liability and related valuation allowance were as follows as of December 31,: 2017 2016 Assets: Net operating loss carryforwards $ 287.0 $ 278.7 Accrued asset retirement obligations 157.0 130.0 Stock-based compensation 3.9 4.3 Unearned revenue 19.3 29.0 Unrealized loss on foreign currency 27.4 26.9 Other accruals and allowances 50.2 45.6 Items not currently deductible and other 28.0 26.9 Liabilities: Depreciation and amortization (1,073.9 ) (942.4 ) Deferred rent (35.9 ) (27.1 ) Other (14.7 ) (9.4 ) Subtotal (551.7 ) (437.5 ) Valuation allowance (142.0 ) (144.4 ) Net deferred tax liabilities $ (693.7 ) $ (581.9 )
Summary of valuation allowance	A summary of the activity in the valuation allowance is as follows: 2017 2016 2015 Balance as of January 1, $ 144.4 $ 137.0 $ 141.2 Additions (1) 11.6 14.1 19.5 Reversals (9.1 ) — — Foreign currency translation (4.9 ) (6.7 ) (23.7 ) Balance as of December 31, $ 142.0 $ 144.4 $ 137.0 _______________ (1) Includes net charges to expense and allowances established through goodwill at acquisition.
Net operating loss carryforwards expire	At December 31, 2017 , the Company had net federal, state and foreign operating loss carryforwards available to reduce future taxable income. If not utilized, the Company’s NOLs expire as follows: Years ended December 31, Federal State Foreign 2018 to 2022 $ — $ 90.7 $ 73.2 2023 to 2027 — 361.3 192.3 2028 to 2032 141.6 85.9 — 2033 to 2037 24.6 122.7 — Indefinite carryforward — — 739.7 Total $ 166.2 $ 660.6 $ 1,005.2
Change in unrecognized tax benefit	A reconciliation of the beginning and ending amount of unrecognized tax benefits are as follows for the years ended December 31,: 2017 2016 2015 Balance at January 1 $ 107.6 $ 28.1 $ 31.9 Additions based on tax positions related to the current year 7.6 82.9 5.0 Additions for tax positions of prior years — — — Foreign currency 1.9 (0.2 ) (5.3 ) Reduction as a result of the lapse of statute of limitations and effective settlements (0.4 ) (3.2 ) (3.5 ) Balance at December 31 $ 116.7 $ 107.6 $ 28.1

STOCK-BASED COMPENSATION (Table

STOCK-BASED COMPENSATION (Tables)	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary of stock-based compensation expenses	During the years ended December 31, 2017, 2016 and 2015, the Company recorded and capitalized the following stock-based compensation expenses: 2017 2016 2015 Stock-based compensation expense $ 108.5 $ 89.9 $ 90.5 Stock-based compensation expense capitalized as property and equipment 1.6 1.4 2.1
Assumptions used to determine the grant date fair value for options granted	Key assumptions used to apply this pricing model were as follows: 2017 2016 2015 Range of risk-free interest rate 1.88%-1.94% 1.00%-1.73% 1.32% - 1.62% Weighted average risk-free interest rate 1.89% 1.44% 1.61% Range of expected life of stock options 5.2 years 4.5 - 5.2 years 4.5 years Range of expected volatility of the underlying stock price 18.95% - 19.45% 20.59% - 21.45% 21.09% - 21.24% Weighted average expected volatility of underlying stock price 19.05% 21.43% 21.09% Range of expected annual dividend yield 2.40% 1.85% - 2.40% 1.50% - 1.85%
Summary of the company's option activity	The Company’s option activity for the year ended December 31, 2017 was as follows: Options Weighted Average Exercise Price Weighted Average Remaining Life (Years) Aggregate Intrinsic Value (in millions) Outstanding as of January 1, 2017 7,269,376 $78.00 Granted 6,534 118.20 Exercised (1,663,001 ) 66.57 Forfeited (55,348 ) 93.09 Expired — — Outstanding as of December 31, 2017 5,557,561 $81.32 6.07 $341.0 Exercisable as of December 31, 2017 3,425,213 $74.47 5.24 $233.6 Vested or expected to vest as of December 31, 2017 5,557,561 $81.32 6.07 $341.0
Schedule of options outstanding	The following table sets forth information regarding options outstanding at December 31, 2017 : Options Outstanding Options Exercisable Outstanding Number of Options Range of Exercise Price Per Share Weighted Average Exercise Price Per Share Weighted Average Remaining Life (Years) Options Exercisable Weighted Average Exercise Price Per Share 593,231 $28.39 - $50.78 $ 45.76 2.59 593,231 $ 45.76 552,500 52.33 - 74.06 61.97 4.17 552,500 61.97 641,460 76.90 - 79.45 76.92 5.15 639,631 76.91 1,242,705 81.18 - 94.23 81.59 6.19 812,744 81.42 2,484,098 94.57 - 94.71 94.62 7.45 820,043 94.59 43,567 96.46 - 121.15 109.38 8.33 7,064 103.90 5,557,561 $28.39 - $121.15 $ 81.32 6.07 3,425,213 $ 74.47
Summary of the company's restricted stock unit activity	The Company’s RSU and PSU activity for the year ended December 31, 2017 was as follows: RSUs Weighted Average Grant Date Fair Value PSUs Weighted Average Grant Date Fair Value Outstanding as of January 1, 2017 (1) 1,663,743 $ 90.76 242,757 $ 93.92 Granted (2) 840,467 114.22 201,274 113.52 Vested (680,610 ) 88.12 — — Forfeited (80,875 ) 101.51 — — Outstanding as of December 31, 2017 1,742,725 $ 102.60 444,031 $ 102.81 Expected to vest as of December 31, 2017 1,742,725 $ 102.60 444,031 $ 102.81 _______________ (1) PSUs represent the shares issuable for the 2015 PSUs (as defined below) at the end of the three -year performance cycle based on achievement against the performance metric for the first and second year’s performance periods, or 73,417 shares, and the target number of shares issuable at the end of the three -year performance period for the 2016 PSUs (as defined below), or 169,340 shares. (2) PSUs represent the shares issuable for the 2015 PSUs at the end of the three-year performance cycle based on exceeding the performance metric for the third year’s performance period, or 46,754 shares, and the target number of shares issuable at the end of the three-year performance cycle for the 2017 PSUs (as defined below), or 154,520 shares.

REDEEMABLE NONCONTROLLING INT47

REDEEMABLE NONCONTROLLING INTERESTS (Tables)	12 Months Ended
	Dec. 31, 2017
Temporary Equity Disclosure [Abstract]
Schedule of Redeemable Noncontrolling Interests	The following is a reconciliation of the changes in the Redeemable noncontrolling interests: Balance as of January 1, 2016 $ — Fair value at acquisition 1,100.9 Net income attributable to noncontrolling interests 13.9 Foreign currency translation adjustment attributable to noncontrolling interests (23.5 ) Balance as of January 1, 2017 $ 1,091.3 Net loss attributable to noncontrolling interests (33.4 ) Foreign currency translation adjustment attributable to noncontrolling interests 68.3 Balance as of December 31, 2017 $ 1,126.2

Equity (Tables)

Equity (Tables)	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Schedule of Dividends Declared	Distributions —During the years ended December 31, 2017 , 2016 and 2015, the Company declared the following cash distributions: For the year ended December 31, 2017 2016 2015 Distribution Aggregate Distribution Aggregate Distribution Aggregate Common Stock $ 2.62 $ 1,122.5 $ 2.17 $ 923.7 $ 1.81 $ 766.4 Series A Preferred Stock $ 2.63 $ 15.8 $ 5.25 $ 31.5 $ 3.94 $ 23.7 Series B Preferred Stock $ 55.00 $ 75.6 $ 55.00 $ 75.6 $ 38.65 $ 53.1
Schedule of Stock by Class	The following table characterizes the tax treatment of distributions declared per share of common stock and Mandatory Convertible Preferred Stock. For the year ended December 31, 2017 2016 2015 Per Share % Per Share % Per Share % Common Stock Ordinary dividend $ 2.6200 (1) 100.00 % $ 2.1700 100.00 % $ 1.2694 70.13 % Capital gains distribution — — — — 0.5406 29.87 Total $ 2.6200 100.00 % $ 2.1700 100.00 % $ 1.8100 100.00 % Series A Preferred Stock Ordinary dividend $ 3.3643 (2) 100.00 % $ 6.4578 (3) 100.00 % $ 3.6818 (4) 70.13 % Capital gains distribution — — — — 1.5682 29.87 Total $ 3.3643 100.00 % $ 6.4578 100.00 % $ 5.2500 100.00 % Series B Preferred Stock (5) Ordinary dividend $ 6.5233 (6) 100.00 % $ 5.5000 100.00 % $ 2.7107 70.13 % Capital gains distribution — — — — 1.1546 29.87 Total $ 6.5233 100.00 % $ 5.5000 100.00 % $ 3.8653 100.00 % _______________ (1) Includes dividend declared on December 6, 2017 of $0.70 per share, which was paid on January 16, 2018 to common stockholders of record at the close of business on December 28, 2017. (2) Includes a deemed distribution as a result of a conversion rate adjustment triggered on April 27, 2017. (3) Includes a deemed distribution as a result of a conversion rate adjustment triggered on June 17, 2016. (4) Includes dividend declared on December 2, 2014 of $1.3125 per share, which was paid on February 16, 2015 to preferred stockholders of record at the close of business on February 1, 2015. (5) Represents the tax treatment on dividends per depositary share, each of which represents a 1/10th interest in a share of Series B Preferred Stock. (6) Includes a deemed distribution as a result of a conversion rate adjustment triggered on April 12, 2017.

EARNINGS PER COMMON SHARE (Tabl

EARNINGS PER COMMON SHARE (Tables)	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Schedule of earnings per basic and diluted by common class	The following table sets forth basic and diluted net income per common share computational data for the years ended December 31, (shares in thousands, except per share data): 2017 2016 2015 Net income attributable to American Tower Corporation stockholders $ 1,238.9 $ 956.4 $ 685.1 Dividends on preferred stock (87.4 ) (107.1 ) (90.2 ) Net income attributable to American Tower Corporation common stockholders 1,151.5 849.3 594.9 Basic weighted average common shares outstanding 428,181 425,143 418,907 Dilutive securities 3,507 4,140 4,108 Diluted weighted average common shares outstanding 431,688 429,283 423,015 Basic net income attributable to American Tower Corporation common stockholders per common share $ 2.69 $ 2.00 $ 1.42 Diluted net income attributable to American Tower Corporation common stockholders per common share $ 2.67 $ 1.98 $ 1.41
Schedule of antidilutive securities excluded from computation of earnings per share	The following shares were not included in the computation of diluted earnings per share because the effect would be anti-dilutive for the years ended December 31, (in thousands, on a weighted average basis): 2017 2016 2015 Restricted stock awards 3 6 — Stock options 4 817 1,606 Preferred stock 14,040 17,509 15,408

COMMITMENTS AND CONTINGENCIES (

COMMITMENTS AND CONTINGENCIES (Tables)	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Future minimum rental payments under non-cancelable operating leases	Future minimum rental payments under non-cancellable operating leases include payments for certain renewal periods at the Company’s option because failure to renew could result in a loss of the applicable communications sites and related revenues from tenant leases, thereby making it reasonably assured that the Company will renew the leases. Such payments at December 31, 2017 are as follows: Year Ending December 31, 2018 $ 924 2019 887 2020 848 2021 811 2022 768 Thereafter 6,533 Total $ 10,771
Future minimum payments under capital leases	Future minimum payments under capital leases in effect at December 31, 2017 were as follows: Year Ending December 31, 2018 $ 34 2019 31 2020 26 2021 21 2022 18 Thereafter 166 Total minimum lease payments 296 Less amounts representing interest (130 ) Present value of capital lease obligations $ 166
Future minimum rental receipts under operating lease agreements	Future minimum rental receipts expected from tenants under non-cancellable operating lease agreements in effect at December 31, 2017 were as follows: Year Ending December 31, 2018 $ 4,927 2019 4,683 2020 4,393 2021 3,957 2022 3,095 Thereafter 11,180 Total $ 32,235

SUPPLEMENTAL CASH FLOW INFORM51

SUPPLEMENTAL CASH FLOW INFORMATION (Tables)	12 Months Ended
	Dec. 31, 2017
Supplemental Cash Flow Elements [Abstract]
Supplemental cash flow information and non-cash investing and financing activities	Supplemental cash flow information and non-cash investing and financing activities are as follows for the years ended December 31,: 2017 2016 2015 Supplemental cash flow information: Cash paid for interest $ 712.1 $ 645.1 $ 578.0 Cash paid for income taxes (net of refunds of $20.7, $19.6 and $7.1, respectively) 136.5 96.2 157.1 Non-cash investing and financing activities: Increase (decrease) in accounts payable and accrued expenses for purchases of property and equipment and construction activities 34.0 (19.0 ) 2.8 Purchases of property and equipment under capital leases 54.8 55.6 36.9 Fair value of debt assumed through acquisitions — 786.9 — Exercise of purchase option for property and equipment for common shares issued — 120.8 — Settlement of accounts receivable related to acquisitions — — 0.9 Conversion of third-party debt to equity 48.2 — —

BUSINESS SEGMENTS (Tables)

BUSINESS SEGMENTS (Tables)	12 Months Ended
	Dec. 31, 2017
Segment Reporting, Measurement Disclosures [Abstract]
Summarized financial information concerning the company's reportable segments	Summarized financial information concerning the Company’s reportable segments for the years ended December 31, 2017 , 2016 and 2015 is shown in the following tables. The “Other” column (i) represents amounts excluded from specific segments, such as business development operations, stock-based compensation expense and corporate expenses included in Selling, general, administrative and development expense; Other operating expenses; Interest income; Interest expense; Gain (loss) on retirement of long-term obligations; and Other income (expense), as the amounts are not utilized in assessing each segment’s performance, and (ii) reconciles segment operating profit to Income from continuing operations before income taxes. Property Total Property Services Other Total Year ended December 31, 2017 U.S. Asia EMEA Latin America Segment revenues $ 3,605.7 $ 1,164.4 $ 626.2 $ 1,169.6 $ 6,565.9 $ 98.0 $ 6,663.9 Segment operating expenses (1) 746.5 649.0 238.3 386.1 2,019.9 33.8 2,053.7 Interest income, TV Azteca, net — — — 10.8 10.8 — 10.8 Segment gross margin 2,859.2 515.4 387.9 794.3 4,556.8 64.2 4,621.0 Segment selling, general, administrative and development expense (1) 151.4 82.4 67.9 77.5 379.2 13.7 392.9 Segment operating profit $ 2,707.8 $ 433.0 $ 320.0 $ 716.8 $ 4,177.6 $ 50.5 $ 4,228.1 Stock-based compensation expense $ 108.5 108.5 Other selling, general, administrative and development expense 138.5 138.5 Depreciation, amortization and accretion 1,715.9 1,715.9 Other expense (2) 1,009.1 1,009.1 Income from continuing operations before income taxes $ 1,256.1 Capital expenditures (3) $ 360.6 $ 118.0 $ 141.7 $ 197.4 $ 817.7 $ — $ 17.7 $ 835.4 _______________ (1) Segment operating expenses and segment selling, general, administrative and development expenses exclude stock-based compensation expense of $2.9 million and $105.6 million , respectively. (2) Primarily includes interest expense. (3) Includes $31.8 million of capital lease payments included in Repayments of notes payable, credit facilities, term loan, senior notes and capital leases in the cash flow from financing activities in the Company’s consolidated statement of cash flows. Property Total Property Services Other Total Year ended December 31, 2016 U.S. Asia EMEA Latin America Segment revenues $ 3,370.1 $ 827.6 $ 529.5 $ 985.9 $ 5,713.1 $ 72.6 $ 5,785.7 Segment operating expenses (1) 733.4 465.9 223.7 338.0 1,761.0 27.0 1,788.0 Interest income, TV Azteca, net — — — 10.9 10.9 — 10.9 Segment gross margin 2,636.7 361.7 305.8 658.8 3,963.0 45.6 4,008.6 Segment selling, general, administrative and development expense (1) 147.6 48.2 60.9 60.7 317.4 12.5 329.9 Segment operating profit $ 2,489.1 $ 313.5 $ 244.9 $ 598.1 $ 3,645.6 $ 33.1 $ 3,678.7 Stock-based compensation expense $ 89.9 89.9 Other selling, general, administrative and development expense 126.0 126.0 Depreciation, amortization and accretion 1,525.6 1,525.6 Other expense (2) 811.3 811.3 Income from continuing operations before income taxes $ 1,125.9 Capital expenditures (3) $ 310.7 $ 115.5 $ 86.1 $ 172.6 $ 684.9 $ — $ 16.5 $ 701.4 _______________ (1) Segment operating expenses and segment selling, general, administrative and development expenses exclude stock-based compensation expense of $2.4 million and $87.5 million , respectively. (2) Primarily includes interest expense. (3) Includes $18.9 million of capital lease payments included in Repayments of notes payable, credit facilities, term loan, senior notes and capital leases in the cash flow from financing activities in the Company’s consolidated statement of cash flows. Property Total Property Other Total Year ended December 31, 2015 U.S. Asia EMEA Latin America Segment revenues $ 3,157.5 $ 242.2 $ 395.1 $ 885.6 $ 4,680.4 $ 91.1 $ 4,771.5 Segment operating expenses (1) 678.5 126.9 163.8 304.6 1,273.8 33.0 1,306.8 Interest income, TV Azteca, net — — — 11.2 11.2 — 11.2 Segment gross margin 2,479.0 115.3 231.3 592.2 3,417.8 58.1 3,475.9 Segment selling, general, administrative and development expense (1) 138.6 22.7 48.7 62.2 272.2 15.7 287.9 Segment operating profit $ 2,340.4 $ 92.6 $ 182.6 $ 530.0 $ 3,145.6 $ 42.4 $ 3,188.0 Stock-based compensation expense $ 90.5 90.5 Other selling, general, administrative and development expense 121.4 121.4 Depreciation, amortization and accretion 1,285.3 1,285.3 Other expense (2) 860.8 860.8 Income from continuing operations before income taxes $ 830.0 Capital expenditures $ 367.7 $ 75.4 $ 66.6 $ 201.8 $ 711.5 $ — $ 17.3 $ 728.8 _______________ (1) Segment operating expenses and segment selling, general, administrative and development expenses exclude stock-based compensation expense of $2.0 million and $88.5 million , respectively. (2) Primarily includes interest expense.
Reconciliation of assets from segments to consolidated	Additional information relating to the total assets of the Company’s operating segments is as follows for the years ended December 31,: 2017 2016 2015 U.S. property $ 19,032.6 $ 18,846.9 $ 19,286.5 Asia property (1) 4,770.8 4,535.3 736.1 EMEA property (1) 3,213.6 2,062.4 2,249.6 Latin America property (1) 5,868.4 4,938.1 4,401.3 Services 42.3 48.3 68.4 Other (2) 286.6 448.2 162.4 Total assets $ 33,214.3 $ 30,879.2 $ 26,904.3 _______________ (1) Balances are translated at the applicable period end exchange rate, which may impact comparability between periods. (2) Balances include corporate assets such as cash and cash equivalents, certain tangible and intangible assets and income tax accounts that have not been allocated to specific segments.
Schedule of disclosure on geographic areas, long-lived assets	Summarized geographic information related to the Company’s operating revenues for the years ended December 31, 2017 , 2016 and 2015 and long-lived assets as of December 31, 2017 and 2016 is as follows: 2017 2016 2015 Operating Revenues: United States $ 3,703.7 $ 3,442.7 $ 3,248.6 Asia (1): India 1,164.4 827.6 242.2 EMEA (1): France 59.5 — — Germany 63.1 60.2 56.0 Ghana 122.9 116.2 94.5 Nigeria 213.9 215.4 109.7 South Africa 106.5 80.0 80.5 Uganda 60.3 57.7 54.4 Latin America (1): Argentina 15.9 1.0 — Brazil 620.1 506.2 408.6 Chile 40.4 33.8 29.7 Colombia 89.3 79.7 78.4 Costa Rica 19.4 19.0 17.2 Mexico 364.3 331.2 340.5 Paraguay 2.7 — — Peru 17.5 15.0 11.2 Total International 2,960.2 2,343.0 1,522.9 Total operating revenues $ 6,663.9 $ 5,785.7 $ 4,771.5 _______________ (1) Balances are translated at the applicable exchange rate, which may impact comparability between periods. 2017 2016 Long-Lived Assets (1): United States $ 16,930.2 $ 16,969.6 Asia (2): India 4,052.6 4,094.2 EMEA (2): France 1,009.6 — Germany 428.0 397.3 Ghana 171.4 192.2 Nigeria 587.2 640.6 South Africa 330.4 271.8 Uganda 136.9 141.5 Latin America (2): Argentina 117.9 137.6 Brazil 2,557.4 2,626.4 Chile 151.2 137.2 Colombia 369.0 272.3 Costa Rica 112.9 117.5 Mexico 1,396.8 797.8 Paraguay 77.5 — Peru 93.7 66.6 Total International 11,592.5 9,893.0 Total long-lived assets $ 28,522.7 $ 26,862.6 _______________ (1) Includes Property and equipment, net, Goodwill and Other intangible assets, net. (2) Balances are translated at the applicable period end exchange rate, which may impact comparability between periods.
Schedule of revenue by major customers	The following tenants within the property and services segments individually accounted for 10% or more of the Company’s consolidated operating revenues for the years ended December 31, is as follows: 2017 2016 2015 AT&T 19 % 21 % 24 % Verizon Wireless 16 % 15 % 16 % Sprint 9 % 11 % 13 % T-Mobile 9 % 9 % 10 %

SELECTED QUARTERLY FINANCIAL 53

SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (Tables)	12 Months Ended
	Dec. 31, 2017
Selected Quarterly Financial Information [Abstract]
Schedule of quarterly financial information	Selected quarterly financial data for the years ended December 31, 2017 and 2016 is as follows (in millions, except per share data): Three Months Ended Year Ended December 31, March 31, June 30, September 30, December 31, 2017: Operating revenues $ 1,616.2 $ 1,662.5 $ 1,680.7 $ 1,704.5 $ 6,663.9 Costs of operations (1) 492.7 517.2 519.8 526.9 2,056.6 Operating income 531.4 576.9 561.1 329.0 1,998.4 Net income 307.4 388.5 334.7 194.8 1,225.4 Net income attributable to American Tower Corporation stockholders 316.1 367.0 317.3 238.5 1,238.9 Dividends on preferred stock (26.8 ) (22.8 ) (18.9 ) (18.9 ) (87.4 ) Net income attributable to American Tower Corporation common stockholders 289.3 344.2 298.4 219.6 1,151.5 Basic net income per share attributable to American Tower Corporation common stockholders 0.68 0.81 0.70 0.51 2.69 Diluted net income per share attributable to American Tower Corporation common stockholders 0.67 0.80 0.69 0.51 2.67 Three Months Ended Year Ended December 31, (2) March 31, June 30, September 30, December 31, 2016: Operating revenues $ 1,289.0 $ 1,442.2 $ 1,514.8 $ 1,539.5 $ 5,785.7 Costs of operations (1) 351.4 459.7 491.2 488.0 1,790.4 Operating income 451.9 432.8 479.1 489.3 1,853.0 Net income 281.3 192.5 263.7 232.9 970.4 Net income attributable to American Tower Corporation stockholders 275.2 187.6 264.5 229.2 956.4 Dividends on preferred stock (26.8 ) (26.8 ) (26.8 ) (26.8 ) (107.1 ) Net income attributable to American Tower Corporation common stockholders 248.4 160.8 237.7 202.4 849.3 Basic net income per share attributable to American Tower Corporation common stockholders 0.59 0.38 0.56 0.48 2.00 Diluted net income per share attributable to American Tower Corporation common stockholders 0.58 0.37 0.55 0.47 1.98 _______________ (1) Represents Operating expenses, exclusive of Depreciation, amortization and accretion, Selling, general, administrative and development expense, and Other operating expenses. (2) The amounts reported for the year ended December 31, 2016 differ slightly from the sum of the quarters due to rounding.

OTHER NON-CURRENT LIABILITIES (

OTHER NON-CURRENT LIABILITIES (Tables)	12 Months Ended
	Dec. 31, 2017
Other Liabilities, Noncurrent [Abstract]
Other non-current liabilities	Other non-current liabilities consisted of the following as of December 31,: 2017 2016 Unearned revenue $ 509.2 $ 457.3 Deferred rent liability 467.0 407.2 Other miscellaneous liabilities 268.0 278.1 Other non-current liabilities $ 1,244.2 $ 1,142.6

BUSINESS AND SUMMARY OF SIGNI55

BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)customersjoint_ventureshares	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Principles of Consolidation and Basis of Presentation:
Number of joint ventures with controlling interests | joint_venture	2
Accounts Receivable and Deferred Rent Asset:
Number of customers, concentration of credit risk | customers	4
Functional Currency:
Foreign currency transaction gain (loss), in AOCI and Income Statement	$ (25.1)
Foreign currency transaction gain (loss), reclassified as AOCI	51.6
Foreign currency transaction (loss)	(26.5)
Property and Equipment:
Labor costs capitalized	50.9	$ 47.7	$ 44.7
Other Comprehensive Income (Loss):
Accumulated other comprehensive loss	(1,978.3)	(1,999.3)
Revenue Recognition:
Straight line revenue	194.4	131.7	155
Rent Expense:
Straight-line ground rent expense	$ 62.3	$ 67.8	$ 56.1
Share-based Compensation:
Shares paid for tax withholding for share based compensation | shares	1,442,506
Retirement Plan:
Employers percentage of employees first 6 percent	100.00%	75.00%	75.00%
Employee maximum annual contribution eligible for match	5.00%	6.00%	6.00%
Company's contribution	$ 11	$ 9.1	$ 7.4
New accounting pronouncement, percent of revenues effected	14.00%
Restricted Stock
Share-based Compensation:
Shares paid for tax withholding for share based compensation | shares	222,751
Restricted Stock | 2007 Plan
Share-based Compensation:
Vesting period	4 years
Performance Shares
Share-based Compensation:
Vesting period	3 years
Performance Shares | 2007 Plan
Share-based Compensation:
Vesting period	3 years
Accumulated Foreign Currency Adjustment Attributable to Parent
Other Comprehensive Income (Loss):
Accumulated other comprehensive loss	$ 2,000	$ 2,000	$ 1,800
Maximum
Goodwill and Other Intangible Assets:
Estimated useful life of respective assets	20 years
Maximum | Performance Shares
Share-based Compensation:
Percentage of potential target shares	200.00%
Minimum
Goodwill and Other Intangible Assets:
Estimated useful life of respective assets	3 years
Minimum | Performance Shares
Share-based Compensation:
Percentage of potential target shares	0.00%
Sales Revenue, Net | Customer Concentration Risk
Accounts Receivable and Deferred Rent Asset:
Concentration risk, percentage	10.00%	10.00%	10.00%
ATC Europe
Principles of Consolidation and Basis of Presentation:
Company's ownership percentage	51.00%
Noncontrolling owners, ownership percentage	49.00%
ATC, TIPL
Principles of Consolidation and Basis of Presentation:
Company's ownership percentage	51.00%
Four Customers | Sales Revenue, Net | Customer Concentration Risk
Accounts Receivable and Deferred Rent Asset:
Concentration risk, percentage	53.00%
Ghana
Principles of Consolidation and Basis of Presentation:
Number of joint ventures with controlling interests | joint_venture	1
Uganda
Principles of Consolidation and Basis of Presentation:
Number of joint ventures with controlling interests | joint_venture	1
Corporate Joint Venture | Ghana
Principles of Consolidation and Basis of Presentation:
Company's ownership percentage	51.00%
Noncontrolling owners, ownership percentage	49.00%
Corporate Joint Venture | Uganda
Principles of Consolidation and Basis of Presentation:
Company's ownership percentage	51.00%
Noncontrolling owners, ownership percentage	49.00%
Corporate Joint Venture | South Africa
Principles of Consolidation and Basis of Presentation:
Company's ownership percentage	75.00%
Noncontrolling owners, ownership percentage	25.00%

BUSINESS AND SUMMARY OF SIGNI56

BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Changes in Allowances) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Doubtful Accounts Receivable [Roll Forward]
Balance as of January 1	$ 45.9	$ 23.1	$ 17.3
Current year increases	87.2	50	19.9
Write-offs, recoveries and other	(2.1)	(27.2)	(14.1)
Balance as of December 31	$ 131	$ 45.9	$ 23.1

BUSINESS AND SUMMARY OF SIGNI57

BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Cash, Cash Equivalents, And Restricted Cash) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Cash and cash equivalents	$ 802.1	$ 787.2	$ 320.7
Restricted cash	152.8	149.3	142.2
Total cash, cash equivalents and restricted cash	$ 954.9	$ 936.5	$ 462.9	$ 473.7

PREPAID AND OTHER CURRENT ASS58

PREPAID AND OTHER CURRENT ASSETS (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Prepaid Expense and Other Assets, Current [Abstract]
Prepaid operating ground leases	$ 148.6	$ 134.2
Prepaid income tax	136.5	127.1
Unbilled receivables	107.9	57.7
Value added tax and other consumption tax receivables	64.2	31.6
Prepaid assets	39.6	36.3
Other miscellaneous current assets	71.8	54.1
Prepaids and other current assets	$ 568.6	$ 441

PROPERTY AND EQUIPMENT (Details

PROPERTY AND EQUIPMENT (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Towers	$ 12,500.5	$ 11,740.5
Equipment	1,423	1,176.3
Buildings and improvements	631.4	621.9
Land and improvements	2,112.9	1,909.7
Construction-in-progress	282.1	203.4
Total	16,949.9	15,651.8
Less accumulated depreciation	(5,848.9)	(5,134.5)
Property and equipment, net	$ 11,101	$ 10,517.3
Minimum
Property, Plant and Equipment [Line Items]
Estimated useful lives	3 years
Maximum
Property, Plant and Equipment [Line Items]
Estimated useful lives	20 years
Tower | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful lives	20 years
Equipment | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful lives	2 years
Equipment | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful lives	15 years
Buildings and Improvements | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful lives	3 years
Buildings and Improvements | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful lives	32 years
Land and Improvements | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful lives	20 years

PROPERTY AND EQUIPMENT (Narrati

PROPERTY AND EQUIPMENT (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 835.5	$ 758.9	$ 661.4
Assets Held under Capital Leases
Property, Plant and Equipment [Line Items]
Capital leases, which are primarily classified as towers or land and improvements	4,944.2	4,735.3
Capital leases, accumulated depreciation	$ 1,370.4	$ 1,198

GOODWILL AND OTHER INTANGIBLE61

GOODWILL AND OTHER INTANGIBLE ASSETS (Rollforward) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Roll Forward]
Goodwill, beginning balance	$ 5,070.7	$ 4,091.9
Additions	486.1	975.7
Effect of foreign currency translation	81.6	3.1
Goodwill, ending balance	5,638.4	5,070.7
Services
Goodwill [Roll Forward]
Goodwill, beginning balance	2	2
Additions	0	0
Effect of foreign currency translation	0	0
Goodwill, ending balance	2	2
U.S. | Property
Goodwill [Roll Forward]
Goodwill, beginning balance	3,379.2	3,379.2
Additions	0	0
Effect of foreign currency translation	0	0
Goodwill, ending balance	3,379.2	3,379.2
Asia | Property
Goodwill [Roll Forward]
Goodwill, beginning balance	1,029.3	170.7
Additions	0.4	881.8
Effect of foreign currency translation	65.3	(23.2)
Goodwill, ending balance	1,095	1,029.3
EMEA | Property
Goodwill [Roll Forward]
Goodwill, beginning balance	150.5	132.6
Additions	220.9	40.4
Effect of foreign currency translation	33.5	(22.5)
Goodwill, ending balance	404.9	150.5
Latin America | Property
Goodwill [Roll Forward]
Goodwill, beginning balance	509.7	407.4
Additions	264.8	53.5
Effect of foreign currency translation	(17.2)	48.8
Goodwill, ending balance	757.3	$ 509.7
2017 Acquisitions
Goodwill [Roll Forward]
Additions	485.1
Goodwill, Impaired, Adjustment to Initial Estimate Amount	$ 1

GOODWILL AND OTHER INTANGIBLE62

GOODWILL AND OTHER INTANGIBLE ASSETS (Changes in the Carrying Value of Goodwill) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	$ 16,083	$ 14,794.8
Accumulated Amortization	(4,299.7)	(3,520.2)
Net Book Value	11,783.3	11,274.6
Acquired network location intangibles
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	4,858.8	4,622.3
Accumulated Amortization	(1,525.3)	(1,280.3)
Net Book Value	$ 3,333.5	3,342
Acquired network location intangibles | Maximum
Finite-Lived Intangible Assets [Line Items]
Intangible assets, useful life	20 years
Acquired tenant-related intangibles
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	$ 11,150.9	10,130.5
Accumulated Amortization	(2,754.7)	(2,224.1)
Net Book Value	$ 8,396.2	7,906.4
Acquired tenant-related intangibles | Minimum
Finite-Lived Intangible Assets [Line Items]
Intangible assets, useful life	15 years
Acquired tenant-related intangibles | Maximum
Finite-Lived Intangible Assets [Line Items]
Intangible assets, useful life	20 years
Acquired licenses and other intangibles
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	$ 58.8	28.1
Accumulated Amortization	(8.1)	(4.8)
Net Book Value	$ 50.7	23.3
Acquired licenses and other intangibles | Minimum
Finite-Lived Intangible Assets [Line Items]
Intangible assets, useful life	3 years
Acquired licenses and other intangibles | Maximum
Finite-Lived Intangible Assets [Line Items]
Intangible assets, useful life	20 years
Economic Rights, TV Azteca
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	$ 14.5	13.9
Accumulated Amortization	(11.6)	(11)
Net Book Value	$ 2.9	$ 2.9
Intangible assets, useful life	70 years

GOODWILL AND OTHER INTANGIBLE63

GOODWILL AND OTHER INTANGIBLE ASSETS (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Acquired Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	$ 785.9	$ 699.8	$ 568.3
Weighted Average
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, remaining amortization period	15 years

GOODWILL AND OTHER INTANGIBLE64

GOODWILL AND OTHER INTANGIBLE ASSETS (Expected Future Amortization Expenses) (Details) $ in Millions	Dec. 31, 2017USD ($)
Goodwill and Intangible Assets Disclosure [Abstract]
2,018	$ 810.5
2,019	806.7
2,020	787.2
2,021	768.7
2,022	$ 766.1

NOTES RECEIVABLE AND OTHER NO65

NOTES RECEIVABLE AND OTHER NON-CURRENT ASSETS (Notes Receivable and Other Non-Current Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Notes Receivable And Other Long Term Assets [Abstract]
Long-term prepaid ground rent	$ 552.8	$ 467.8
Notes receivable	83.7	83.7
Other miscellaneous assets	313.6	290
Notes receivable and other non-current assets	$ 950.1	$ 841.5

NOTES RECEIVABLE AND OTHER NO66

NOTES RECEIVABLE AND OTHER NON-CURRENT ASSETS (Narrative) (Details) $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2000USD ($)	Dec. 31, 2000USD ($)communications_site	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Economic Rights, TV Azteca
Notes Receivable And Other NonCurrent Assets [Line Items]
Term of the loan, years		70 years
Loan prepayment without penalty, period, years	50 years
Number of broadcast towers | communications_site		190
Commercial rights, annual payment	$ 1.5	$ 1.5
Percentage of the revenues from leasing of towers		100.00%
Capital lease, asset	18.6	$ 18.6
Capital lease, liability	18.6	18.6
Capital lease asset and discount on note	30.2	30.2
TV Azteca
Notes Receivable And Other NonCurrent Assets [Line Items]
Loans receivable	$ 119.8	$ 119.8	$ 91.8	$ 91.8
Loan interest rate		13.11%
Term of the loan, years		70 years
Loan prepayment without penalty, period, years		50 years
Loans receivable, net of discount				$ 82.9
Repayment of Principal | TV Azteca
Notes Receivable And Other NonCurrent Assets [Line Items]
Loans receivable			$ 28

ACQUISITIONS (Merger and Integr

ACQUISITIONS (Merger and Integration Expenses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Combinations [Abstract]
Acquisition and merger related expenses	$ 16.3	$ 15.9	$ 18.8
Integration costs	$ 11.5	$ 9.9	$ 18.1

ACQUISITIONS (Narrative) (Detai

ACQUISITIONS (Narrative) (Details) concrete_pole in Thousands, € in Millions, $ in Millions, ₨ in Billions	Nov. 17, 2017USD ($)	Nov. 17, 2017concrete_pole	Nov. 17, 2017route_miles_of_fiber	Nov. 13, 2017USD ($)site	Nov. 13, 2017INR (₨)site	Feb. 15, 2017USD ($)tower	Feb. 15, 2017EUR (€)tower	Apr. 21, 2016USD ($)site	Apr. 21, 2016INR (₨)	Dec. 31, 2017USD ($)communications_site	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Apr. 21, 2016INR (₨)site
Business Acquisition [Line Items]
Proceeds from previous acquisition										$ 22.2
Business combination, pro forma information, revenue of acquiree since acquisition date, actual										82.1
Business combination pro forma information gross margin of acquiree since acquisition date actual										59.3
Remainder purchase price										0	$ 4.7	$ 5,059.5
Accounts payable										142.9	118.7
Fair value of debt assumed through acquisitions										0	786.9	0
Redeemable noncontrolling interest										1,126.2	1,091.3	$ 0
Contingent consideration										10.1
Other Acquisitions 2017
Business Acquisition [Line Items]
Asset acquisition, consideration transferred										814
Accounts payable										$ 22.5
Communication Sites | Other Acquisitions 2017
Business Acquisition [Line Items]
Number of communications sites acquired | communications_site										2,453
FPS Towers
Business Acquisition [Line Items]
Percentage of voting interest acquired						100.00%	100.00%
Purchase price						$ 771.2	€ 727.2
Number of communications sites acquired | tower						2,500	2,500
Fair value of debt assumed through acquisitions						$ 0
FPS Towers | Affiliated Entity
Business Acquisition [Line Items]
Loan portion of consideration transferred						238.6	€ 225
Remainder purchase price						$ 532.6	€ 502.2
Entities With Urban Telecommunications Assets In Mexico
Business Acquisition [Line Items]
Percentage of voting interest acquired	100.00%	100.00%	100.00%
Purchase price	$ 505.8
Number of assets acquired		50	2,100
Viom Transaction
Business Acquisition [Line Items]
Percentage of voting interest acquired								51.00%					51.00%
Purchase price										$ 1,148.5
Fair value of debt assumed through acquisitions								$ 800	₨ 52.3	786.8
Viom Transaction | Preliminary Purchase Price Allocation
Business Acquisition [Line Items]
Purchase price								1,100	₨ 76.4
Viom Transaction | Mandatorily Redeemable Preferred Stock
Business Acquisition [Line Items]
Redeemable noncontrolling interest								$ 25.1					₨ 1.7
Other Acquisition 2016
Business Acquisition [Line Items]
Purchase price										304.4	304.4
Number of communications sites acquired | site								891					891
Accounts payable											12.1
Fair value of debt assumed through acquisitions										$ 0
Contingent consideration											$ 8.8
Idea Cellular Limited
Business Acquisition [Line Items]
Percentage of voting interest acquired				100.00%	100.00%
Purchase price				$ 611.4	₨ 40
Number of communications sites acquired | site				9,900	9,900
Vodafone
Business Acquisition [Line Items]
Purchase price				$ 588.4	₨ 38.5
Number of communications sites acquired | site				10,235	10,235

ACQUISITIONS (Schedule of Aggre

ACQUISITIONS (Schedule of Aggregate Purchase Consideration Paid and the Amount of Assets Acquired) (Details) € in Millions, $ in Millions, ₨ in Billions	Nov. 17, 2017USD ($)	Feb. 15, 2017USD ($)tower	Feb. 15, 2017EUR (€)	Apr. 21, 2016USD ($)site	Apr. 21, 2016INR (₨)site	Dec. 31, 2017USD ($)communications_site	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Business Acquisition [Line Items]
Goodwill						$ 5,638.4	$ 5,070.7	$ 4,091.9
Debt assumed						0	(786.9)	$ 0
Accounts payable						$ 142.9	118.7
Tenant-related intangible assets | Maximum
Business Acquisition [Line Items]
Intangible assets, useful life						20 years
Network location intangible assets | Maximum
Business Acquisition [Line Items]
Intangible assets, useful life						20 years
FPS Towers
Business Acquisition [Line Items]
Current assets		$ 34.5
Non-current assets		15
Property and equipment		122.9
Current liabilities		(29)
Deferred tax liability		(135.4)
Other non-current liabilities		(19.9)
Net assets acquired		550.3
Goodwill		220.9
Fair value of net assets acquired		771.2
Debt assumed		0
Purchase price		$ 771.2	€ 727.2
Number of communications sites acquired | tower		2,500
FPS Towers | Tenant-related intangible assets
Business Acquisition [Line Items]
Intangible assets		$ 440.7
FPS Towers | Tenant-related intangible assets | Maximum
Business Acquisition [Line Items]
Intangible assets, useful life		20 years	20 years			20 years
FPS Towers | Network location intangible assets
Business Acquisition [Line Items]
Intangible assets		$ 113
FPS Towers | Network location intangible assets | Maximum
Business Acquisition [Line Items]
Intangible assets, useful life		20 years	20 years			20 years
FPS Towers | Other intangible assets
Business Acquisition [Line Items]
Intangible assets		$ 8.5
Entities With Urban Telecommunications Assets In Mexico
Business Acquisition [Line Items]
Purchase price	$ 505.8
Other Acquisitions 2017
Business Acquisition [Line Items]
Current assets						$ 12.7
Non-current assets						19.7
Property and equipment						290
Current liabilities						(10.5)
Deferred tax liability						(2.7)
Other non-current liabilities						(14.2)
Net assets acquired						814
Goodwill						0
Fair value of net assets acquired						814
Debt assumed						0
Purchase price						814
Other Acquisitions 2017 | Tenant-related intangible assets
Business Acquisition [Line Items]
Intangible assets						364.7
Other Acquisitions 2017 | Network location intangible assets
Business Acquisition [Line Items]
Intangible assets						154.3
Other Acquisitions 2017 | Other intangible assets
Business Acquisition [Line Items]
Intangible assets						0
Viom Transaction
Business Acquisition [Line Items]
Current assets						281.9
Non-current assets						52.3
Property and equipment						705.8
Current liabilities						(201.1)
Deferred tax liability						(619.1)
Other non-current liabilities						(101.8)
Net assets acquired						2,154
Goodwill						882.2
Fair value of net assets acquired						3,036.2
Debt assumed				$ (800)	₨ (52.3)	(786.8)
Redeemable noncontrolling interests						(1,100.9)
Purchase price						1,148.5
Viom Transaction | Tenant-related intangible assets
Business Acquisition [Line Items]
Intangible assets						$ 1,369.6
Viom Transaction | Tenant-related intangible assets | Maximum
Business Acquisition [Line Items]
Intangible assets, useful life						20 years
Viom Transaction | Network location intangible assets
Business Acquisition [Line Items]
Intangible assets						$ 666.4
Viom Transaction | Network location intangible assets | Maximum
Business Acquisition [Line Items]
Intangible assets, useful life						20 years
Other Acquisition 2016
Business Acquisition [Line Items]
Current assets						$ 24.5
Non-current assets						2.3
Property and equipment						81.5
Current liabilities						(14.8)
Deferred tax liability						(43.4)
Other non-current liabilities						(29.4)
Net assets acquired						209.9
Goodwill						94.5
Fair value of net assets acquired						304.4
Debt assumed						0
Redeemable noncontrolling interests						0
Purchase price						304.4	304.4
Number of communications sites acquired | site				891	891
Accounts payable							12.1
Other Acquisition 2016 | Tenant-related intangible assets
Business Acquisition [Line Items]
Intangible assets						$ 105.6
Other Acquisition 2016 | Tenant-related intangible assets | Maximum
Business Acquisition [Line Items]
Intangible assets, useful life						20 years
Other Acquisition 2016 | Network location intangible assets
Business Acquisition [Line Items]
Intangible assets						$ 83.6
Other Acquisition 2016 | Network location intangible assets | Maximum
Business Acquisition [Line Items]
Intangible assets, useful life						20 years
Scenario, Previously Reported | Entities With Urban Telecommunications Assets In Mexico
Business Acquisition [Line Items]
Current assets						$ 44.4
Non-current assets						0
Property and equipment						94
Current liabilities						(28.8)
Deferred tax liability						(38.8)
Other non-current liabilities						(4.5)
Net assets acquired						241.6
Goodwill						264.2
Fair value of net assets acquired						505.8
Debt assumed						0
Purchase price						505.8
Scenario, Previously Reported | Entities With Urban Telecommunications Assets In Mexico | Tenant-related intangible assets
Business Acquisition [Line Items]
Intangible assets						153.3
Scenario, Previously Reported | Entities With Urban Telecommunications Assets In Mexico | Network location intangible assets
Business Acquisition [Line Items]
Intangible assets						0
Scenario, Previously Reported | Entities With Urban Telecommunications Assets In Mexico | Other intangible assets
Business Acquisition [Line Items]
Intangible assets						22
Scenario, Previously Reported | Viom Transaction
Business Acquisition [Line Items]
Current assets							276.6
Non-current assets							57.6
Property and equipment							702
Current liabilities							(195.9)
Deferred tax liability							(619.1)
Other non-current liabilities							(102.8)
Net assets acquired							2,154.4
Goodwill							881.8
Fair value of net assets acquired							3,036.2
Debt assumed							(786.8)
Redeemable noncontrolling interests							(1,100.9)
Purchase price							1,148.5
Scenario, Previously Reported | Viom Transaction | Tenant-related intangible assets
Business Acquisition [Line Items]
Intangible assets							$ 1,369.6
Scenario, Previously Reported | Viom Transaction | Tenant-related intangible assets | Maximum
Business Acquisition [Line Items]
Intangible assets, useful life							20 years
Scenario, Previously Reported | Viom Transaction | Network location intangible assets
Business Acquisition [Line Items]
Intangible assets							$ 666.4
Scenario, Previously Reported | Viom Transaction | Network location intangible assets | Maximum
Business Acquisition [Line Items]
Intangible assets, useful life							20 years
Scenario, Previously Reported | Other Acquisition 2016
Business Acquisition [Line Items]
Current assets							$ 25.5
Non-current assets							2.3
Property and equipment							81.5
Current liabilities							(14.8)
Deferred tax liability							(43.8)
Other non-current liabilities							(29.4)
Net assets acquired							210.5
Goodwill							93.9
Fair value of net assets acquired							304.4
Debt assumed							0
Redeemable noncontrolling interests							0
Purchase price							304.4
Scenario, Previously Reported | Other Acquisition 2016 | Tenant-related intangible assets
Business Acquisition [Line Items]
Intangible assets							$ 105.6
Scenario, Previously Reported | Other Acquisition 2016 | Tenant-related intangible assets | Maximum
Business Acquisition [Line Items]
Intangible assets, useful life							20 years
Scenario, Previously Reported | Other Acquisition 2016 | Network location intangible assets
Business Acquisition [Line Items]
Intangible assets							$ 83.6
Scenario, Previously Reported | Other Acquisition 2016 | Network location intangible assets | Maximum
Business Acquisition [Line Items]
Intangible assets, useful life							20 years
Other Acquisitions 2017
Business Acquisition [Line Items]
Accounts payable						$ 22.5
Other Acquisitions 2017 | Tenant-related intangible assets | Maximum
Business Acquisition [Line Items]
Intangible assets, useful life						20 years
Other Acquisitions 2017 | Network location intangible assets | Maximum
Business Acquisition [Line Items]
Intangible assets, useful life						20 years
Communication Sites | Other Acquisitions 2017
Business Acquisition [Line Items]
Number of communications sites acquired | communications_site						2,453
Communication Sites | Peru | Other Acquisitions 2017
Business Acquisition [Line Items]
Number of communications sites acquired | communications_site						127

ACQUISITIONS (Pro Forma Informa

ACQUISITIONS (Pro Forma Information) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Business Combinations [Abstract]
Pro forma revenues	$ 6,240.6	$ 6,775.3
Pro forma net income attributable to American Tower Corporation common stockholders	$ 822.8	$ 1,145.5
Pro forma net income per common share amounts:
Basic net income attributable to American Tower Corporation (in dollars per share)	$ 1.94	$ 2.68
Diluted net income attributable to American Tower Corporation (in dollars per share)	$ 1.92	$ 2.65

ACQUISITIONS (Schedule Of Busin

ACQUISITIONS (Schedule Of Business Acquisitions By Acquisition Contingent Consideration) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)
Business Acquisition [Line Items]
Contingent consideration, maximum potential value	$ 10.1
Contingent consideration	10.1
Contingent consideration, additions	0
Contingent consideration, settlements	0
Change in fair value	(6.3)
Business combination, contingent consideration arrangements, range of outcomes, value, low	9.1
Colombia
Business Acquisition [Line Items]
Contingent consideration, maximum potential value	0
Contingent consideration	0
Contingent consideration, additions	0
Contingent consideration, settlements	0
Change in fair value	(5.4)
Ghana
Business Acquisition [Line Items]
Contingent consideration, maximum potential value	0.6
Contingent consideration	0.6
Contingent consideration, additions	0
Contingent consideration, settlements	0
Change in fair value	0
South Africa
Business Acquisition [Line Items]
Contingent consideration, maximum potential value	9.1
Contingent consideration	9.1
Contingent consideration, additions	0
Contingent consideration, settlements	0
Change in fair value	(0.9)
U.S.
Business Acquisition [Line Items]
Contingent consideration, maximum potential value	0.4
Contingent consideration	0.4
Contingent consideration, additions	0
Contingent consideration, settlements	0
Change in fair value	$ 0

ACCRUED EXPENSES (Details)

ACCRUED EXPENSES (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Accrued Liabilities, Current [Abstract]
Accrued property and real estate taxes	$ 154.4	$ 138.4
Payroll and related withholdings	82.2	76.1
Amounts payable to tenants	60.8	32.3
Accrued rent	54	51
Accrued income tax payable	15.3	11.6
Accrued pass-through costs	59.7	68.5
Accrued construction costs	31.9	28.6
Accrued pass-through taxes	25.3	1
Other accrued expenses	370.7	213
Accrued expenses	$ 854.3	$ 620.5

LONG-TERM OBLIGATIONS (Long-Ter

LONG-TERM OBLIGATIONS (Long-Term Financing Arrangements) (Details) - USD ($)	Dec. 31, 2017	Dec. 08, 2017	Jul. 31, 2017	Jun. 30, 2017	Apr. 06, 2017	Feb. 10, 2017	Dec. 31, 2016	May 29, 2015	Jan. 10, 2014	Aug. 19, 2013	Mar. 31, 2013
Debt Instrument [Line Items]
Long-term debt	$ 20,205,100,000
Other long-term debt	165,500,000						$ 135,900,000
Debt, long-term and short-term, combined amount	20,205,100,000						18,533,500,000
Less current portion long-term obligations	(774,800,000)						(238,800,000)
Long-term debt, excluding current maturities	19,430,300,000						18,294,700,000
Parent
Debt Instrument [Line Items]
Long-term debt	16,494,500,000						14,418,600,000
Subsidiaries
Debt Instrument [Line Items]
Long-term debt	3,545,100,000						3,979,000,000
Credit Facility 2013 | Revolving Credit Facility
Debt Instrument [Line Items]
Long-term line of credit	$ 2,075,600,000						540,000,000
Debt instrument, interest rate, effective percentage	2.649%
Credit Facility 2014 | Revolving Credit Facility
Debt Instrument [Line Items]
Long-term line of credit	$ 495,000,000						1,385,000,000
Debt instrument, interest rate, effective percentage	2.82%
Unsecured Debt
Debt Instrument [Line Items]
Long-term debt	$ 246,100,000						286,000,000
Unsecured Debt | Term Loan 2013
Debt Instrument [Line Items]
Long-term debt	$ 994,500,000						993,900,000
Debt instrument, interest rate, effective percentage	2.79%
Unsecured Debt | India Indebtedness
Debt Instrument [Line Items]
Long-term debt	$ 512,600,000						549,500,000
Unsecured Debt | Shareholder Loans
Debt Instrument [Line Items]
Long-term debt	100,600,000						151,100,000
Senior Notes | 4.500% senior notes
Debt Instrument [Line Items]
Long-term debt	0						998,700,000
Long-term debt, stated interest rate			4.50%
Senior Notes | 3.40% senior notes
Debt Instrument [Line Items]
Long-term debt	$ 999,800,000						999,700,000
Long-term debt, stated interest rate	3.40%								3.40%	3.40%
Senior Notes | 7.25% Senior notes
Debt Instrument [Line Items]
Long-term debt	$ 0						297,000,000
Long-term debt, stated interest rate						7.25%
Senior Notes | 2.800% senior notes
Debt Instrument [Line Items]
Long-term debt	$ 746,300,000						744,900,000
Long-term debt, stated interest rate	2.80%
Senior Notes | 5.05% senior notes
Debt Instrument [Line Items]
Long-term debt	$ 698,000,000						697,400,000
Long-term debt, stated interest rate	5.05%
Senior Notes | 3.300% senior notes
Debt Instrument [Line Items]
Long-term debt	$ 746,000,000						744,800,000
Long-term debt, stated interest rate	3.30%
Senior Notes | 3.450% senior notes
Debt Instrument [Line Items]
Long-term debt	$ 645,100,000						643,800,000
Long-term debt, stated interest rate	3.45%
Senior Notes | 5.900% senior notes
Debt Instrument [Line Items]
Long-term debt	$ 497,800,000						497,300,000
Long-term debt, stated interest rate	5.90%
Senior Notes | 2.250% senior notes
Debt Instrument [Line Items]
Long-term debt	$ 572,400,000						572,800,000
Long-term debt, stated interest rate	2.25%
Senior Notes | 4.70% senior notes
Debt Instrument [Line Items]
Long-term debt	$ 696,700,000						696,000,000
Long-term debt, stated interest rate	4.70%
Senior Notes | 3.50% senior notes
Debt Instrument [Line Items]
Long-term debt	$ 990,900,000						989,300,000
Long-term debt, stated interest rate	3.50%
Senior Notes | 3.000% senior notes
Debt Instrument [Line Items]
Long-term debt	$ 692,500,000						0
Long-term debt, stated interest rate	3.00%	3.00%
Senior Notes | 5.00% senior notes
Debt Instrument [Line Items]
Long-term debt	$ 1,002,400,000						1,002,700,000
Long-term debt, stated interest rate	5.00%								5.00%	5.00%
Senior Notes | 1.375% senior notes
Debt Instrument [Line Items]
Long-term debt	$ 589,100,000						0
Long-term debt, stated interest rate	1.375%				1.375%
Senior Notes | 4.000% senior notes
Debt Instrument [Line Items]
Long-term debt	$ 741,000,000						740,000,000
Long-term debt, stated interest rate	4.00%
Senior Notes | 4.400% senior notes
Debt Instrument [Line Items]
Long-term debt	$ 495,600,000						495,200,000
Long-term debt, stated interest rate	4.40%
Senior Notes | 3.375% senior notes
Debt Instrument [Line Items]
Long-term debt	$ 984,800,000						983,400,000
Long-term debt, stated interest rate	3.375%
Senior Notes | 3.125% senior notes
Debt Instrument [Line Items]
Long-term debt	$ 397,100,000						396,700,000
Long-term debt, stated interest rate	3.125%
Senior Notes | 3.55% senior notes
Debt Instrument [Line Items]
Long-term debt	$ 742,800,000						0
Long-term debt, stated interest rate	3.55%			3.55%
Senior Notes | 3.600% senior notes
Debt Instrument [Line Items]
Long-term debt	$ 691,100,000						0
Long-term debt, stated interest rate	3.60%	3.60%
Secured Debt | Commercial Mortgage Pass Through Certificates Series 2013
Debt Instrument [Line Items]
Long-term debt											$ 1,800,000,000
Secured Debt | Commercial Mortgage Pass Through Certificates Series 2013 | Secured Tower Revenue Securities, Series 2013-1A
Debt Instrument [Line Items]
Long-term debt	$ 499,800,000						498,600,000
Long-term debt, stated interest rate	1.551%
Secured Debt | Commercial Mortgage Pass Through Certificates Series 2013 | Secured Tower Revenue Securities, Series 2013-2A
Debt Instrument [Line Items]
Long-term debt	$ 1,291,800,000						1,290,300,000
Long-term debt, stated interest rate	3.07%
Secured Debt | Series 2015-1 Class A
Debt Instrument [Line Items]
Long-term debt	$ 348,000,000						347,100,000	$ 350,000,000
Long-term debt, stated interest rate	2.35%
Secured Debt | Series 2015-2 Class A
Debt Instrument [Line Items]
Long-term debt	$ 520,100,000						519,400,000	$ 525,000,000
Long-term debt, stated interest rate	3.482%
Secured Debt | GTP Notes
Debt Instrument [Line Items]
Long-term debt	$ 0						179,500,000
Secured Debt | Secured Cellular Site Revenue Notes
Debt Instrument [Line Items]
Long-term debt	0						133,000,000
Mandatorily Redeemable Preferred Stock | Viom preferred shares
Debt Instrument [Line Items]
Long-term debt	$ 26,100,000						$ 24,500,000
Long-term debt, stated interest rate	10.25%
Minimum | Secured Debt | India Indebtedness
Debt Instrument [Line Items]
Long-term debt, stated interest rate	7.90%
Maximum | Secured Debt | India Indebtedness
Debt Instrument [Line Items]
Long-term debt, stated interest rate	9.55%

LONG-TERM OBLIGATIONS (Narrativ

LONG-TERM OBLIGATIONS (Narrative) (Details) ₨ / shares in Units, ₨ in Millions, BRL in Millions	Dec. 08, 2017USD ($)	Jul. 31, 2017USD ($)	Jun. 30, 2017USD ($)	Apr. 06, 2017USD ($)	Apr. 06, 2017EUR (€)	Feb. 15, 2017USD ($)	Feb. 10, 2017USD ($)	May 29, 2015USD ($)tower	Mar. 31, 2013USD ($)tower	Dec. 31, 2017USD ($)shares	Dec. 31, 2016USD ($)quarter	Dec. 31, 2015USD ($)	Dec. 31, 2017INR (₨)shares	Dec. 31, 2017BRLshares	Apr. 06, 2017EUR (€)	Mar. 02, 2017shares	Dec. 31, 2016₨ / shares	Dec. 31, 2016BRLquarter	Nov. 30, 2014BRL
Debt Instrument [Line Items]
Borrowings under credit facilities										$ 5,359,400,000	$ 2,446,800,000	$ 6,126,600,000
Proceeds from issuance of senior notes, net										2,674,000,000	3,236,400,000	1,492,300,000
Loss on retirement of long-term obligations										70,200,000	(1,200,000)	79,600,000
Payment for early retirement of long-term obligations										75,300,000	100,000	$ 85,700,000
Long-term debt										20,205,100,000
Capital lease obligations										$ 165,500,000	135,900,000
Capital lease obligation and notes payable interest rates ranging minimum										3.53%			3.53%	3.53%
Capital lease obligation and notes payable interest rates ranging maximum										9.20%			9.20%	9.20%
Capital lease obligation and notes payable interest mature in periods minimum (less than)										1 year
Capital lease obligation and notes payable interest mature in periods maximum										70 years
Secured Tower Revenue Securities, Series 2013-1A
Debt Instrument [Line Items]
Period during which no prepayment consideration is due										12 months
Secured Tower Revenue Securities, Series 2013-2A
Debt Instrument [Line Items]
Period during which no prepayment consideration is due										18 months
BR Towers Debentures
Debt Instrument [Line Items]
Issuer shares used in securitization, percent										100.00%
Revolving Credit Facility | Credit Facility 2013
Debt Instrument [Line Items]
Line of credit facility, remaining borrowing capacity										$ 2,750,000,000
Borrowings under credit facilities										3,800,000,000
Repayments of lines of credit										2,300,000,000
Principal amount											2,075,600,000
Revolving Credit Facility | Credit Facility 2014
Debt Instrument [Line Items]
Line of credit facility, remaining borrowing capacity										2,000,000,000
Borrowings under credit facilities										815,000,000
Repayments of lines of credit										1,700,000,000
Principal amount											495,000,000
Swingline Loan | Credit Facility 2014
Debt Instrument [Line Items]
Line of credit facility, remaining borrowing capacity										50,000,000
Multicurrency Borrowings | Revolving Credit Facility | Credit Facility 2013
Debt Instrument [Line Items]
Line of credit facility, capacity available for specific purpose other than for trade purchases										1,000,000,000
Letter of Credit | Revolving Credit Facility | Credit Facility 2013
Debt Instrument [Line Items]
Line of credit facility, capacity available for specific purpose other than for trade purchases										200,000,000
Letter of Credit | Revolving Credit Facility | Credit Facility 2014
Debt Instrument [Line Items]
Line of credit facility, capacity available for specific purpose other than for trade purchases										200,000,000
Swingline Loan | Revolving Credit Facility | Credit Facility 2013
Debt Instrument [Line Items]
Line of credit facility, capacity available for specific purpose other than for trade purchases										$ 50,000,000
London Interbank Offered Rate (LIBOR) | Revolving Credit Facility | Credit Facility 2013
Debt Instrument [Line Items]
Basis spread on variable rate										1.125%
London Interbank Offered Rate (LIBOR) | Revolving Credit Facility | Credit Facility 2014
Debt Instrument [Line Items]
Basis spread on variable rate										1.25%
London Interbank Offered Rate (LIBOR) | Revolving Credit Facility | Term Loan 2013
Debt Instrument [Line Items]
Basis spread on variable rate										1.25%
Base Rate | Revolving Credit Facility | Credit Facility 2013
Debt Instrument [Line Items]
Basis spread on variable rate										0.75%
Minimum | London Interbank Offered Rate (LIBOR) | Revolving Credit Facility | Credit Facility 2013
Debt Instrument [Line Items]
Basis spread on variable rate										0.875%
Maximum | London Interbank Offered Rate (LIBOR) | Revolving Credit Facility | Credit Facility 2013
Debt Instrument [Line Items]
Basis spread on variable rate										1.75%
Unsecured Debt
Debt Instrument [Line Items]
Long-term debt										$ 246,100,000	286,000,000
Unsecured Debt | Term Loan 2013
Debt Instrument [Line Items]
Principal amount											1,000,000,000
Long-term debt										$ 994,500,000	$ 993,900,000
Unsecured Debt | BR Towers Debentures
Debt Instrument [Line Items]
Long-term debt | BRL																			BRL 300
Unsecured Debt | Minimum | London Interbank Offered Rate (LIBOR) | Term Loan 2013
Debt Instrument [Line Items]
Basis spread on variable rate										1.00%
Unsecured Debt | Maximum | London Interbank Offered Rate (LIBOR) | Term Loan 2013
Debt Instrument [Line Items]
Basis spread on variable rate										2.00%
Unsecured Debt | Maximum | Base Rate | Term Loan 2013
Debt Instrument [Line Items]
Basis spread on variable rate										1.00%
Senior Notes
Debt Instrument [Line Items]
Proceeds from issuance of senior notes, net	$ 1,382,900,000
Redemption price percentage										100.00%
Repurchase price as percentage of principal											101.00%							101.00%
Maximum adjusted EBITDA											3.5							3.5
Senior Notes | 1.375% senior notes
Debt Instrument [Line Items]
Long-term debt, stated interest rate				1.375%						1.375%			1.375%	1.375%	1.375%
Principal amount				$ 532,200,000						$ 600,240,096					€ 500,000,000
Proceeds from issuance of senior notes, net				$ 521,400,000	€ 489,800,000
Long-term debt										$ 589,100,000	$ 0
Senior Notes | 3.55% senior notes
Debt Instrument [Line Items]
Long-term debt, stated interest rate			3.55%							3.55%			3.55%	3.55%
Principal amount			$ 750,000,000							$ 750,000,000
Proceeds from issuance of senior notes, net			$ 741,800,000
Long-term debt										$ 742,800,000	0
Senior Notes | 3.000% senior notes
Debt Instrument [Line Items]
Long-term debt, stated interest rate	3.00%									3.00%			3.00%	3.00%
Principal amount	$ 700,000,000									$ 700,000,000
Long-term debt										$ 692,500,000	0
Senior Notes | 3.600% senior notes
Debt Instrument [Line Items]
Long-term debt, stated interest rate	3.60%									3.60%			3.60%	3.60%
Principal amount	$ 700,000,000									$ 700,000,000
Long-term debt										691,100,000	0
Senior Notes | 7.25% Senior notes
Debt Instrument [Line Items]
Long-term debt, stated interest rate							7.25%
Redemption price, percentage of principal amount redeemed							112.0854%
Repayments of debt including interest							$ 341,400,000
Repayment of interest on debt							5,100,000
Loss on retirement of long-term obligations							39,200,000
Payment for early retirement of long-term obligations							$ 36,300,000
Long-term debt										0	297,000,000
Senior Notes | 4.500% senior notes
Debt Instrument [Line Items]
Long-term debt, stated interest rate		4.50%
Redemption price, percentage of principal amount redeemed		101.351%
Repayments of debt including interest		$ 1,000,000,000
Repayment of interest on debt		2,000,000
Loss on retirement of long-term obligations		14,100,000
Payment for early retirement of long-term obligations		$ 13,500,000
Long-term debt										$ 0	$ 998,700,000
Secured Debt | Commercial Mortgage Pass Through Certificates Series 2013
Debt Instrument [Line Items]
Proceeds from issuance of senior notes, net									$ 1,780,000,000
Long-term debt									$ 1,800,000,000
Number of broadcast and wireless communications towers | tower									5,178
Weighted average life									8 years 7 months 6 days
Weighted average interest rate									2.648%
Cash trap debt service credit ratio											1.30							1.30
Debt covenant, number of consecutive quarters required for release of funds | quarter											2							2
Minimum debt service credit ratio											1.15							1.15
Debt covenant, threshold for majority of holders in event of default										50.00%
Restricted cash and cash equivalents											$ 90,400,000
Secured Debt | Series 2015-1 Class A
Debt Instrument [Line Items]
Long-term debt, stated interest rate										2.35%			2.35%	2.35%
Long-term debt								$ 350,000,000		$ 348,000,000	347,100,000
Secured Debt | Series 2015-2 Class A
Debt Instrument [Line Items]
Long-term debt, stated interest rate										3.482%			3.482%	3.482%
Long-term debt								$ 525,000,000		$ 520,100,000	519,400,000
Secured Debt | Commercial Mortgage Pass Through Certificates Series 2015
Debt Instrument [Line Items]
Number of broadcast and wireless communications towers | tower								3,583
Weighted average life								8 years 1 month 6 days
Weighted average interest rate								3.029%
Restricted cash and cash equivalents											16,900,000
Secured Debt | GTP Notes
Debt Instrument [Line Items]
Loss on retirement of long-term obligations						$ 1,800,000
Payment for early retirement of long-term obligations						7,200,000
Long-term debt										0	179,500,000
Repayments of long-term debt						173,500,000
Secured Debt | Secured Cellular Site Revenue Notes
Debt Instrument [Line Items]
Loss on retirement of long-term obligations						14,500,000
Payment for early retirement of long-term obligations						18,300,000
Long-term debt										0	133,000,000
Repayments of long-term debt						$ 129,000,000
Secured Debt | Viom Term Loan
Debt Instrument [Line Items]
Long-term debt										$ 418,700,000			₨ 26,740
Secured Debt | Viom Debenture
Debt Instrument [Line Items]
Long-term debt, stated interest rate										9.55%			9.55%	9.55%
Long-term debt										$ 93,900,000			₨ 6,000
Secured Debt | BR Towers Debentures
Debt Instrument [Line Items]
Long-term debt, stated interest rate										7.40%			7.40%	7.40%
Long-term debt										$ 92,700,000	101,000,000			BRL 306.8				BRL 329.3
Secured Debt | Base Rate | Viom Debenture
Debt Instrument [Line Items]
Basis spread on variable rate										0.60%
Basis spread on variable rate, reset rate										10.00%
Basis spread on variable rate, reset period one										36 months
Basis spread on variable rate, reset period two										48 months
Secured Debt | Minimum | Viom Term Loan
Debt Instrument [Line Items]
Debt Instrument, Term										1 year
Long-term debt, stated interest rate										7.90%			7.90%	7.90%
Prepayment penalty, percent of principal										1.00%
Payment terms, repayment period										6 months
Secured Debt | Minimum | Base Rate | Viom Debenture
Debt Instrument [Line Items]
Basis spread on variable rate, reset threshold										9.75%
Secured Debt | Maximum | Viom Term Loan
Debt Instrument [Line Items]
Debt Instrument, Term										10 years
Long-term debt, stated interest rate										8.65%			8.65%	8.65%
Prepayment penalty, percent of principal										2.00%
Payment terms, repayment period										36 months
Secured Debt | Maximum | Base Rate | Viom Debenture
Debt Instrument [Line Items]
Basis spread on variable rate, reset threshold										10.25%
Mandatorily Redeemable Preferred Stock | Viom preferred shares
Debt Instrument [Line Items]
Long-term debt, stated interest rate										10.25%			10.25%	10.25%
Long-term debt										$ 26,100,000	$ 24,500,000
Preferred stock, shares issued | shares																166,666,666
Preferred stock, shares outstanding | shares										166,666,666			166,666,666	166,666,666
Financial instruments subject to mandatory redemption, settlement terms, maximum amount										$ 26,100,000			₨ 1,670
Preferred stock, redemption price per share | ₨ / shares																	₨ 10
Preferred stock, dividend rate, percentage										10.25%
Interest Rate Swap | Senior Notes | 3.000% senior notes
Debt Instrument [Line Items]
Interest rate swap agreements										$ 500,000,000
Derivative, notional amount										$ 800,000
Derivative, fixed interest rate										2.49%			2.49%	2.49%

Long-Term Obligations (Schedule

Long-Term Obligations (Schedule of Line of Credit Facilities) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017renewal_period	Dec. 31, 2016USD ($)
Line of Credit Facility [Line Items]
Annual renewal periods, number | renewal_period	2
Credit Facility 2013 | Revolving Credit Facility
Line of Credit Facility [Line Items]
Principal amount		$ 2,075.6
Letters of credit, amount outstanding		4
Line of credit facility, commitment fee percentage	0.125%
Credit Facility 2014 | Revolving Credit Facility
Line of Credit Facility [Line Items]
Principal amount		495
Letters of credit, amount outstanding		6.3
Line of credit facility, commitment fee percentage	0.15%
Term Loan 2013 | Unsecured Debt
Line of Credit Facility [Line Items]
Principal amount		1,000
Letters of credit, amount outstanding		$ 0
London Interbank Offered Rate (LIBOR) | Credit Facility 2013 | Revolving Credit Facility
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.125%
London Interbank Offered Rate (LIBOR) | Credit Facility 2014 | Revolving Credit Facility
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.25%
London Interbank Offered Rate (LIBOR) | Term Loan 2013 | Revolving Credit Facility
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.25%

Long-Term Obligations (Schedu76

Long-Term Obligations (Schedule of Debt Discounts) (Details) - Senior Notes	Dec. 31, 2017USD ($)	Dec. 08, 2017USD ($)	Jun. 30, 2017USD ($)	Apr. 06, 2017USD ($)	Apr. 06, 2017EUR (€)	Dec. 31, 2016USD ($)	Jan. 10, 2014	Aug. 19, 2013
3.40% senior notes
Debt Instrument [Line Items]
Principal amount	$ 1,000,000,000
Debt Instrument, unamortized discount (premium) and debt issuance costs, net	$ (200,000)					$ (300,000)
Long-term debt, stated interest rate	3.40%						3.40%	3.40%
2.800% senior notes
Debt Instrument [Line Items]
Principal amount	$ 750,000,000
Debt Instrument, unamortized discount (premium) and debt issuance costs, net	$ (3,700,000)					(5,100,000)
Long-term debt, stated interest rate	2.80%
5.05% senior notes
Debt Instrument [Line Items]
Principal amount	$ 700,000,000
Debt Instrument, unamortized discount (premium) and debt issuance costs, net	$ (2,000,000)					(2,600,000)
Long-term debt, stated interest rate	5.05%
3.300% senior notes
Debt Instrument [Line Items]
Principal amount	$ 750,000,000
Debt Instrument, unamortized discount (premium) and debt issuance costs, net	$ (4,000,000)					(5,200,000)
Long-term debt, stated interest rate	3.30%
3.450% senior notes
Debt Instrument [Line Items]
Principal amount	$ 650,000,000
Debt Instrument, unamortized discount (premium) and debt issuance costs, net	$ (4,900,000)					(6,200,000)
Long-term debt, stated interest rate	3.45%
5.900% senior notes
Debt Instrument [Line Items]
Principal amount	$ 500,000,000
Debt Instrument, unamortized discount (premium) and debt issuance costs, net	$ (2,200,000)					(2,700,000)
Long-term debt, stated interest rate	5.90%
2.250% senior notes
Debt Instrument [Line Items]
Principal amount	$ 600,000,000
Debt Instrument, unamortized discount (premium) and debt issuance costs, net	$ (27,600,000)					(27,200,000)
Long-term debt, stated interest rate	2.25%
Interest rate swap agreements	$ 23,700,000					22,300,000
4.70% senior notes
Debt Instrument [Line Items]
Principal amount	700,000,000
Debt Instrument, unamortized discount (premium) and debt issuance costs, net	$ (3,300,000)					(4,000,000)
Long-term debt, stated interest rate	4.70%
3.50% senior notes
Debt Instrument [Line Items]
Principal amount	$ 1,000,000,000
Debt Instrument, unamortized discount (premium) and debt issuance costs, net	(9,100,000)					(10,700,000)
3.000% senior notes
Debt Instrument [Line Items]
Principal amount	700,000,000	$ 700,000,000
Debt Instrument, unamortized discount (premium) and debt issuance costs, net	$ (7,500,000)					0
Long-term debt, stated interest rate	3.00%	3.00%
5.00% senior notes
Debt Instrument [Line Items]
Principal amount	$ 1,000,000,000
Debt Instrument, unamortized discount (premium) and debt issuance costs, net	$ 2,400,000					2,700,000
Long-term debt, stated interest rate	5.00%						5.00%	5.00%
1.375% senior notes
Debt Instrument [Line Items]
Principal amount	$ 600,240,096			$ 532,200,000	€ 500,000,000
Debt Instrument, unamortized discount (premium) and debt issuance costs, net	$ (11,100,000)					0
Long-term debt, stated interest rate	1.375%			1.375%	1.375%
4.000% senior notes
Debt Instrument [Line Items]
Principal amount	$ 750,000,000
Debt Instrument, unamortized discount (premium) and debt issuance costs, net	$ (9,000,000)					(10,000,000)
Long-term debt, stated interest rate	4.00%
4.400% senior notes
Debt Instrument [Line Items]
Principal amount	$ 500,000,000
Debt Instrument, unamortized discount (premium) and debt issuance costs, net	$ (4,400,000)					(4,800,000)
Long-term debt, stated interest rate	4.40%
3.375% senior notes
Debt Instrument [Line Items]
Principal amount	$ 1,000,000,000
Debt Instrument, unamortized discount (premium) and debt issuance costs, net	$ (15,200,000)					(16,600,000)
Long-term debt, stated interest rate	3.375%
3.125% senior notes
Debt Instrument [Line Items]
Principal amount	$ 400,000,000
Debt Instrument, unamortized discount (premium) and debt issuance costs, net	$ (2,900,000)					(3,300,000)
Long-term debt, stated interest rate	3.125%
3.55% senior notes
Debt Instrument [Line Items]
Principal amount	$ 750,000,000		$ 750,000,000
Debt Instrument, unamortized discount (premium) and debt issuance costs, net	$ (7,200,000)					0
Long-term debt, stated interest rate	3.55%		3.55%
3.600% senior notes
Debt Instrument [Line Items]
Principal amount	$ 700,000,000	$ 700,000,000
Debt Instrument, unamortized discount (premium) and debt issuance costs, net	$ (8,900,000)					$ 0
Long-term debt, stated interest rate	3.60%	3.60%
Interest Rate Swap | 2.250% senior notes
Debt Instrument [Line Items]
Derivative, notional amount	$ 600,000,000
Interest Rate Swap | 3.000% senior notes
Debt Instrument [Line Items]
Interest rate swap agreements	500,000,000
Derivative, notional amount	$ 800,000

Long-Term Obligations (India In

Long-Term Obligations (India Indebtedness) (Details) ₨ in Millions, $ in Millions	Dec. 31, 2017USD ($)	Dec. 31, 2017INR (₨)
Debt Instrument [Line Items]
Long-term debt	$ 20,205.1
Secured Debt | Viom Term Loan
Debt Instrument [Line Items]
Long-term debt	418.7	₨ 26,740
Secured Debt | Viom Debenture
Debt Instrument [Line Items]
Long-term debt	$ 93.9	₨ 6,000
Long-term debt, stated interest rate	9.55%	9.55%
Line of Credit | Viom Working Capital Facilities
Debt Instrument [Line Items]
Long-term debt	$ 0	₨ 0
Minimum | Secured Debt | Viom Term Loan
Debt Instrument [Line Items]
Long-term debt, stated interest rate	7.90%	7.90%
Minimum | Line of Credit | Viom Working Capital Facilities
Debt Instrument [Line Items]
Long-term debt, stated interest rate	8.05%	8.05%
Maximum | Secured Debt | Viom Term Loan
Debt Instrument [Line Items]
Long-term debt, stated interest rate	8.65%	8.65%
Maximum | Line of Credit | Viom Working Capital Facilities
Debt Instrument [Line Items]
Long-term debt, stated interest rate	8.75%	8.75%

Long-Term Obligations (Other Su

Long-Term Obligations (Other Subsidiary Debt) (Details) ZAR in Millions, COP in Millions, $ in Millions

Dec. 23, 2016ZAR

Dec. 31, 2017USD ($)

Dec. 31, 2016USD ($)

Dec. 31, 2015USD ($)

Dec. 31, 2017COP

Dec. 31, 2017BRL

Dec. 31, 2017ZAR

Dec. 31, 2016COP

Dec. 31, 2016BRL

Dec. 31, 2016ZAR

Dec. 31, 2015BRL

Jan. 23, 2015ZAR

Nov. 30, 2014BRL

Debt Instrument [Line Items]

Long-term debt

$ 20,205.1

Borrowings under credit facilities

5,359.4

$ 2,446.8

$ 6,126.6

Secured Debt | BR Towers Debentures

Debt Instrument [Line Items]

Long-term debt

$ 92.7

101

BRL 306,800,000

BRL 329,300,000

Long-term debt, stated interest rate

7.40%

7.40%

7.40%

7.40%

Revolving Credit Facility | South African Facility

Debt Instrument [Line Items]

Borrowings under credit facilities | ZAR

ZAR 500

Revolving Credit Facility | Colombian Credit Facility

Debt Instrument [Line Items]

Long-term debt | COP

COP 200,000

Unsecured Debt

Debt Instrument [Line Items]

Long-term debt

$ 246.1

286

Unsecured Debt | BR Towers Debentures

Debt Instrument [Line Items]

Long-term debt | BRL

BRL 300,000,000

Unsecured Debt | Brazil Credit Facility

Debt Instrument [Line Items]

Line of credit facility, maximum borrowing capacity | BRL

BRL 271,000,000

Revolving Credit Facility | South African Facility

Debt Instrument [Line Items]

Long-term debt | ZAR

ZAR 830

Long-term debt, stated interest rate

9.108%

9.108%

9.108%

9.108%

Long-term line of credit

$ 69.9

84.3

ZAR 866

ZAR 1,157.6

Revolving Credit Facility | Colombian Credit Facility

Debt Instrument [Line Items]

Long-term debt, stated interest rate

8.513%

8.513%

8.513%

8.513%

Long-term line of credit

$ 46.5

56.1

COP 138,740.3

COP 168,286.5

Revolving Credit Facility | Brazil Credit Facility

Debt Instrument [Line Items]

Long-term line of credit

$ 37

$ 44.6

BRL 122,400,000

BRL 145,400,000

Long-Term Obligations (Schedu79

Long-Term Obligations (Schedule of Shareholder Loans) (Details) GHS in Millions, $ in Millions, UGX in Billions	Dec. 31, 2017USD ($)	Dec. 31, 2017GHS	Dec. 31, 2017UGX	Jan. 01, 2017USD ($)	Jan. 01, 2017UGX	Dec. 31, 2016USD ($)
Debt Instrument [Line Items]
Long-term debt	$ 20,205.1
Unsecured Debt
Debt Instrument [Line Items]
Long-term debt	246.1					$ 286
Unsecured Debt | Ghana Loan
Debt Instrument [Line Items]
Long-term debt	$ 66.5	GHS 300.9				71
Long-term debt, stated interest rate	21.87%	21.87%	21.87%
Unsecured Debt | Uganda Loan
Debt Instrument [Line Items]
Long-term debt	$ 34.1		UGX 124.1	$ 80		$ 80.1
Long-term debt, stated interest rate	16.80%	16.80%	16.80%	16.80%	16.80%
Unsecured Debt | Uganda Replacement Loan
Debt Instrument [Line Items]
Long-term debt				$ 31.8	UGX 114.5

Long-Term Obligations (Maturiti

Long-Term Obligations (Maturities of Long Term Debt) (Details) $ in Millions	Dec. 31, 2017USD ($)
Debt Disclosure [Abstract]
2,018	$ 775
2,019	1,192.6
2,020	2,034
2,021	4,051.2
2,022	1,388.7
Thereafter	10,908.1
Total cash obligations	20,349.6
Unamortized discounts, premiums and debt issuance costs and fair value adjustments, net	(144.5)
Balance as of December 31, 2017	$ 20,205.1

ASSET RETIREMENT OBLIGATIONS (C

ASSET RETIREMENT OBLIGATIONS (Carrying Value of Asset Retirement Obligations) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance as of January 1,	$ 965.5	$ 856.9
Additions	33.4	64.1
Accretion expense	94.5	67
Revisions in estimates	86.6	(21.1)
Settlements	(4.7)	(1.4)
Balance as of December 31,	1,175.3	965.5
Asset retirement obligation, foreign currency translation loss	$ 13	$ 9.6

ASSET RETIREMENT OBLIGATIONS (N

ASSET RETIREMENT OBLIGATIONS (Narrative) (Details) $ in Billions	Dec. 31, 2017USD ($)
Asset Retirement Obligation [Abstract]
Estimated undiscounted future cash outlay for asset retirement obligations	$ 3

FAIR VALUE MEASUREMENTS (Assets

FAIR VALUE MEASUREMENTS (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jul. 01, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	$ 1	$ 4
Fair value of embedded derivative asset				$ 14.6
Acquisition-related contingent consideration	10.1
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	1	4
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of debt related to interest rate swap agreements	(24.5)	(22.3)
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swap agreements	29	24.7
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Colombian Credit Facility | Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset, fair value, gross asset	0	0
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of embedded derivative asset	12.4	13.3
Acquisition-related contingent consideration	$ 10.1	$ 15.4	$ 12.4

FAIR VALUE MEASUREMENTS (Narrat

FAIR VALUE MEASUREMENTS (Narrative) (Details) COP in Billions	12 Months Ended
	Dec. 31, 2017USD ($)interest_rate_swap	Dec. 31, 2016USD ($)	Dec. 31, 2017COPinterest_rate_swap	Dec. 08, 2017	Jul. 01, 2015USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of embedded derivative asset					$ 14,600,000
Business combination, contingent consideration arrangements, range of outcomes, value, low	$ 9,100,000
Business combination, contingent consideration arrangements, range of outcomes, value, high	10,100,000
Assets held and used, original carrying value	21,700,000,000	$ 12,700,000,000
Impairment of long-lived assets held for use	211,400,000	28,500,000
Debt, long-term and short-term, combined amount	20,205,100,000	18,533,500,000
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value loss recognized in other expense	900,000
Estimate of Fair Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt, fair value	20,600,000,000	18,800,000,000
Estimate of Fair Value Measurement | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt, fair value	13,300,000,000	11,800,000,000
Estimate of Fair Value Measurement | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt, fair value	$ 7,300,000,000	7,000,000,000
Senior Notes | 3.000% senior notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative, stated interest rate	3.00%		3.00%	3.00%
Senior Notes | 2.250% senior notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swap agreements	$ 23,700,000	22,300,000
Derivative, stated interest rate	2.25%		2.25%
Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative, fair value	$ 28,500,000	$ 24,700,000
Interest Rate Swap | Senior Notes | 3.000% senior notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swaps entered into | interest_rate_swap	3		3
Interest rate swap agreements	$ 500,000,000
Derivative, notional amount	$ 800,000
Derivative, fixed interest rate	2.49%		2.49%
Interest Rate Swap | Senior Notes | 2.250% senior notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swaps entered into | interest_rate_swap	3		3
Derivative, notional amount	$ 600,000,000
Fair value loss recognized in other expense	1,500,000
Interest Rate Swap | Revolving Credit Facility | Colombian Credit Facility
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset, notional amount	$ 23,500,000		COP 70
Derivative, fixed interest rate	5.74%		5.74%
Colombia
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Business combination, contingent consideration arrangements, range of outcomes, value, high	$ 0
Colombia | Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative, fair value	$ 500,000

FAIR VALUE MEASUREMENTS (Contin

FAIR VALUE MEASUREMENTS (Contingent Consideration) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Change in fair value	$ (6.3)
Balance as of December 31	10.1
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance as of January 1	15.4	$ 12.4
Additions	0	8.8
Settlements	0	(0.3)
Change in fair value	(6.3)	(6.4)
Foreign currency translation adjustment	1	0.9
Balance as of December 31	$ 10.1	$ 15.4

INCOME TAXES (Income Tax Provis

INCOME TAXES (Income Tax Provision from Continuing Operations) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current:
Federal	$ (0.1)	$ (26.5)	$ (73.9)
State	(3.8)	(2)	(21.2)
Foreign	(113.4)	(100.1)	(55.1)
Deferred:
Federal	0.2	(0.6)	9.1
State	1	(0.3)	0
Foreign	85.4	(26)	(16.9)
Income tax provision	$ (30.7)	$ (155.5)	$ (158)

INCOME TAXES (Narrative) (Detai

INCOME TAXES (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Contingency [Line Items]
Tax Cuts and Jobs Act of 2017, change in tax rate, deferred tax asset, income tax expense	$ 2,400,000
Valuation allowance	142,000,000	$ 144,400,000	$ 137,000,000	$ 141,200,000
Unrecognized tax benefits that would impact the ETR	105,800,000	102,900,000
Additions based on tax positions related to the current year	7,600,000	82,900,000	5,000,000
Decrease in unrecognized tax benefits, lapse of applicable statute of limitations	400,000	3,200,000	3,500,000
Unrecognized tax benefits, income tax penalties and interest	5,000,000	9,200,000	3,200,000
Unrecognized Tax Benefits, Decrease In Income Tax Penalties And Interest Expense Resulting From Expiration Of Applicable Statute Of Limitations	600,000	3,400,000	$ 3,100,000
Unrecognized tax benefits, income tax penalties and interest accrued	29,000,000	$ 24,300,000
Minimum
Income Tax Contingency [Line Items]
Decrease in unrecognized tax benefits is reasonably possible	0
Maximum
Income Tax Contingency [Line Items]
Decrease in unrecognized tax benefits is reasonably possible	11,800,000
Ghana Revenue Authority | Foreign
Income Tax Contingency [Line Items]
Tax benefit	$ 17,800,000

INCOME TAXES (Reconciliation Be

INCOME TAXES (Reconciliation Between the U.S. Statutory Rate and the Effective Rate from Continuing Operations) (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Statutory tax rate	35.00%	35.00%	35.00%
Tax adjustment related to REIT	(35.00%)	(35.00%)	(35.00%)
Foreign taxes	1.00%	5.00%	3.00%
Foreign withholding taxes	3.00%	4.00%	3.00%
Uncertain tax positions	0.00%	5.00%	0.00%
Changes in tax laws	(2.00%)	0.00%	2.00%
MIPT tax election	0.00%	0.00%	11.00%
Effective tax rate	2.00%	14.00%	19.00%

INCOME TAXES (Components of Inc

INCOME TAXES (Components of Income from Continuing Operations Before Income Taxes and Income on Equity Method Investments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
United States	$ 971.2	$ 882.6	$ 785.2
Foreign	284.9	243.3	44.8
INCOME FROM CONTINUING OPERATIONS BEFORE INCOME TAXES	$ 1,256.1	$ 1,125.9	$ 830

INCOME TAXES (Components of the

INCOME TAXES (Components of the Net Deferred Tax Asset and Related Valuation Allowance) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 287	$ 278.7
Accrued asset retirement obligations	157	130
Stock-based compensation	3.9	4.3
Unearned revenue	19.3	29
Unrealized loss on foreign currency	27.4	26.9
Other accruals and allowances	50.2	45.6
Items not currently deductible and other	28	26.9
Depreciation and amortization	(1,073.9)	(942.4)
Deferred rent	(35.9)	(27.1)
Other	(14.7)	(9.4)
Subtotal	(551.7)	(437.5)
Valuation allowance	(142)	(144.4)	$ (137)	$ (141.2)
Net deferred tax liabilities	$ (693.7)	$ (581.9)

INCOME TAXES (Valuation Allowan

INCOME TAXES (Valuation Allowance Activity) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance as of January 1,	$ 144.4	$ 137	$ 141.2
Additions	11.6	14.1	19.5
Reversals	(9.1)	0	0
Foreign currency translation	(4.9)	(6.7)	(23.7)
Balance as of December 31,	$ 142	$ 144.4	$ 137

INCOME TAXES (Net Operating Los

INCOME TAXES (Net Operating Loss Carryforwards Expire) (Details) $ in Millions	Dec. 31, 2017USD ($)
Federal
Operating Loss Carryforwards [Line Items]
2018 to 2022	$ 0
2023 to 2027	0
2028 to 2032	141.6
2033 to 2037	24.6
Indefinite carryforward	0
Total	166.2
State
Operating Loss Carryforwards [Line Items]
2018 to 2022	90.7
2023 to 2027	361.3
2028 to 2032	85.9
2033 to 2037	122.7
Indefinite carryforward	0
Total	660.6
Foreign
Operating Loss Carryforwards [Line Items]
2018 to 2022	73.2
2023 to 2027	192.3
2028 to 2032	0
2033 to 2037	0
Indefinite carryforward	739.7
Total	$ 1,005.2

INCOME TAXES (Reconciliation of

INCOME TAXES (Reconciliation of the Beginning and Ending Amount of Unrecognized Tax Benefits) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at January 1	$ 107.6	$ 28.1	$ 31.9
Additions based on tax positions related to the current year	7.6	82.9	5
Additions for tax positions of prior years	0	0	0
Foreign currency	1.9
Foreign currency		(0.2)	(5.3)
Reduction as a result of the lapse of statute of limitations and effective settlements	(0.4)	(3.2)	(3.5)
Balance at December 31	$ 116.7	$ 107.6	$ 28.1

STOCK-BASED COMPENSATION (Narra

STOCK-BASED COMPENSATION (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Discount from market price	15.00%
Stock-based compensation expense	$ 108.5	$ 89.9	$ 90.5
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation expense	$ 101.6
Expected recognition of stock award compensation expense weighted average period in years	2 years
Total fair value of restricted stock units vested during period	$ 78.3
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Total unrecognized compensation expense	$ 22.1
Expected recognition of stock award compensation expense weighted average period in years	2 years
Third year performance grant, shares issuable based on first and second years			73,417
Three year performance grant, shares granted	154,520	169,340	70,135
Three year performance grant, shares issuable based on third year			46,754
Stock-based compensation expense	$ 23.6
Performance Shares | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of potential target shares	0.00%
Performance Shares | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of potential target shares	200.00%
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant date fair value (in dollars per share)	$ 16.84	$ 14.60	$ 15.06
Intrinsic value of stock options exercised	$ 100.3	$ 77.6	$ 32.1
Total unrecognized compensation expense	$ 12.4
Expected recognition of stock award compensation expense weighted average period in years	2 years
Employee service share-based compensation, cash received from exercise of stock options	$ 110.7
2007 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares issuable under stock incentive plan	8,500,000
2007 Plan | Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	4 years
2007 Plan | Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
2007 Plan | Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period	10 years

STOCK-BASED COMPENSATION (Summa

STOCK-BASED COMPENSATION (Summary of Stock-Based Compensation Expenses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-based compensation expense	$ 108.5	$ 89.9	$ 90.5
Stock-based compensation expense capitalized as property and equipment	$ 1.6	$ 1.4	$ 2.1

STOCK-BASED COMPENSATION (Assum

STOCK-BASED COMPENSATION (Assumptions Used to Determine the Grant Date Fair Value for Options Granted) (Details) - Employee Stock Option	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Minimum range of risk-free interest rate	1.88%	1.00%	1.32%
Maximum range of risk-free interest rate	1.94%	1.73%	1.62%
Weighted average risk-free interest rate	1.89%	1.44%	1.61%
Expected life of stock options	5 years 2 months 5 days		4 years 4 months 52 days
Minimum range of expected volatility of underlying stock price	18.95%	20.59%	21.09%
Maximum range of expected volatility of underlying stock price	19.45%	21.45%	21.24%
Weighted average expected volatility of underlying stock price	19.05%	21.43%	21.09%
Range of expected annual dividend yield	2.40%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life of stock options		4 years 6 months
Range of expected annual dividend yield		1.85%	1.50%
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life of stock options		5 years 2 months 12 days
Range of expected annual dividend yield		2.40%	1.85%

STOCK-BASED COMPENSATION (Sum97

STOCK-BASED COMPENSATION (Summary of the Company's Option Activity) (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)$ / sharesshares
Weighted Average Exercise Price
Weighted Average Remaining Life, Outstanding	6 years 26 days
Weighted Average Remaining Life, Exercisable	5 years 2 months 28 days
Weighted Average Remaining Life, Vested or expected to vest	6 years 26 days
Employee Stock Option
Options
Outstanding (in shares) | shares	7,269,376
Granted (in shares) | shares	6,534
Exercised (in shares) | shares	(1,663,001)
Forfeited (in shares) | shares	(55,348)
Expired (in shares) | shares	0
Outstanding (in shares) | shares	5,557,561
Exercisable (in shares) | shares	3,425,213
Vested or expected to vest (in shares) | shares	5,557,561
Weighted Average Exercise Price
Outstanding (in dollars per share) | $ / shares	$ 78
Granted (in dollars per share) | $ / shares	118.20
Exercised (in dollars per share) | $ / shares	66.57
Forfeited (in dollars per share) | $ / shares	93.09
Expired (in dollars per share) | $ / shares	0
Outstanding (in dollars per share) | $ / shares	81.32
Exercisable (in dollars per share) | $ / shares	74.47
Vested or expected to vest (in dollars per share) | $ / shares	$ 81.32
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Aggregate intrinsic value, outstanding | $	$ 341
Aggregate intrinsic value, exercisable | $	233.6
Aggregate intrinsic value, vested or expected to vest | $	$ 341

STOCK-BASED COMPENSATION (Sched

STOCK-BASED COMPENSATION (Schedule of Options Outstanding) (Details)	12 Months Ended
	Dec. 31, 2017$ / sharesshares
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Outstanding Number of Options (in shares) | shares	5,557,561
Exercise price per share, minimum	$ 28.39
Exercise price per share, maximum	121.15
Weighted Average Exercise Price Per Share (in dollars per share)	$ 81.32
Weighted Average Remaining Life	6 years 26 days
Options Exercisable (in shares) | shares	3,425,213
Weighted Average Exercise Price Per Share (in dollars per share)	$ 74.47
$28.39 - $50.78
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Outstanding Number of Options (in shares) | shares	593,231
Exercise price per share, minimum	$ 28.39
Exercise price per share, maximum	50.78
Weighted Average Exercise Price Per Share (in dollars per share)	$ 45.76
Weighted Average Remaining Life	2 years 7 months 2 days
Options Exercisable (in shares) | shares	593,231
Weighted Average Exercise Price Per Share (in dollars per share)	$ 45.76
52.33 - 74.06
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Outstanding Number of Options (in shares) | shares	552,500
Exercise price per share, minimum	$ 52.33
Exercise price per share, maximum	74.06
Weighted Average Exercise Price Per Share (in dollars per share)	$ 61.97
Weighted Average Remaining Life	4 years 2 months 1 day
Options Exercisable (in shares) | shares	552,500
Weighted Average Exercise Price Per Share (in dollars per share)	$ 61.97
76.90 - 79.45
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Outstanding Number of Options (in shares) | shares	641,460
Exercise price per share, minimum	$ 76.90
Exercise price per share, maximum	79.45
Weighted Average Exercise Price Per Share (in dollars per share)	$ 76.92
Weighted Average Remaining Life	5 years 1 month 24 days
Options Exercisable (in shares) | shares	639,631
Weighted Average Exercise Price Per Share (in dollars per share)	$ 76.91
81.18 - 94.23
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Outstanding Number of Options (in shares) | shares	1,242,705
Exercise price per share, minimum	$ 81.18
Exercise price per share, maximum	94.23
Weighted Average Exercise Price Per Share (in dollars per share)	$ 81.59
Weighted Average Remaining Life	6 years 2 months 9 days
Options Exercisable (in shares) | shares	812,744
Weighted Average Exercise Price Per Share (in dollars per share)	$ 81.42
94.57 - 94.71
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Outstanding Number of Options (in shares) | shares	2,484,098
Exercise price per share, minimum	$ 94.57
Exercise price per share, maximum	94.71
Weighted Average Exercise Price Per Share (in dollars per share)	$ 94.62
Weighted Average Remaining Life	7 years 5 months 12 days
Options Exercisable (in shares) | shares	820,043
Weighted Average Exercise Price Per Share (in dollars per share)	$ 94.59
96.46 - 121.15
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Outstanding Number of Options (in shares) | shares	43,567
Exercise price per share, minimum	$ 96.46
Exercise price per share, maximum	121.15
Weighted Average Exercise Price Per Share (in dollars per share)	$ 109.38
Weighted Average Remaining Life	8 years 3 months 29 days
Options Exercisable (in shares) | shares	7,064
Weighted Average Exercise Price Per Share (in dollars per share)	$ 103.90

STOCK-BASED COMPENSATION (Sum99

STOCK-BASED COMPENSATION (Summary of the Company's Restricted Stock Unit Activity) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017
Restricted stock awards
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]
Outstanding (in shares)	1,663,743
Granted (in shares)	840,467
Vested (in shares)	(680,610)
Forfeited (in shares)	(80,875)
Outstanding (in shares)	1,663,743	1,742,725
Expected to vest, net of estimated forfeitures (in shares)		1,742,725
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Outstanding (in dollars per share)	$ 90.76
Granted (in dollars per share)	114.22
Vested (in dollars per share)	88.12
Forfeitures (in dollars per share)	101.51
Outstanding (in dollars per share)	$ 102.60
Expected to vest, net of estimated forfeitures (in dollars per share)		$ 102.60
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]
Outstanding (in shares)	242,757
Granted (in shares)	201,274
Vested (in shares)	0
Forfeited (in shares)	0
Outstanding (in shares)	242,757	444,031
Expected to vest, net of estimated forfeitures (in shares)		444,031
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Outstanding (in dollars per share)	$ 93.92
Granted (in dollars per share)	113.52
Vested (in dollars per share)	0
Forfeitures (in dollars per share)	0
Outstanding (in dollars per share)	$ 102.81
Expected to vest, net of estimated forfeitures (in dollars per share)		$ 102.81

REDEEMABLE NONCONTROLLING IN100

REDEEMABLE NONCONTROLLING INTERESTS (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2017₨ / shares
Temporary Equity Disclosure [Abstract]
Temporary equity redemption price per share (in INR per share) | ₨ / shares			₨ 216
Increase (Decrease) in Temporary Equity [Roll Forward]
Beginning balance	$ 1,091.3	$ 0
Fair value at acquisition		1,100.9
Net income attributable to noncontrolling interests	(33.4)	13.9
Foreign currency translation adjustment attributable to noncontrolling interests	68.3	(23.5)
Ending balance	$ 1,126.2	$ 1,091.3

Equity (Narrative) (Details)

Equity (Narrative) (Details) - USD ($)	Feb. 15, 2018	May 15, 2017	May 31, 2017	Mar. 31, 2015	May 31, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Mar. 31, 2011
Class of Stock [Line Items]
Proceeds from stock options and stock purchase plan						$ 119,700,000	$ 92,500,000	$ 50,700,000
Issuance of common stock—stock purchase plan						9,000,000	$ 7,500,000	$ 6,600,000
Treasury stock activity						$ 766,300,000
Treasury stock, shares						8,909,000,000	2,810,000
Treasury stock						$ 974,000,000	$ 207,700,000
Accrued dividend RSU						$ 10,100,000
Series B Preferred Stock
Class of Stock [Line Items]
Preferred stock, depositary shares, percentage of preferred shares						0.1
Convertible Preferred Stock Subject to Mandatory Redemption | Series A Preferred Stock
Class of Stock [Line Items]
Preferred stock, dividend rate, percentage						5.25%	5.25%
Convertible Preferred Stock Subject to Mandatory Redemption | Series B Preferred Stock
Class of Stock [Line Items]
Preferred stock, dividend rate, percentage						5.50%
Dividends Declared and Paid
Class of Stock [Line Items]
Paid dividend RSU						$ 3,000,000
Preferred stock | Convertible Preferred Stock Subject to Mandatory Redemption | Series A Preferred Stock
Class of Stock [Line Items]
Issuance of stock (shares)					6,000,000
Preferred stock, dividend rate, percentage					5.25%
Preferred stock, par value					$ 0.01	$ 0.01	$ 0.01
Conversion factor, preferred stock (shares)						0.9337
Shares converted (shares)						5,602,153
Dividends		$ 7,900,000
Conversion of preferred stock (shares)						(6,000,000)
Preferred stock | Convertible Preferred Stock Subject to Mandatory Redemption | Series B Preferred Stock
Class of Stock [Line Items]
Issuance of stock (shares)				1,375,000				1,375,000
Preferred stock, dividend rate, percentage				5.50%
Preferred stock, par value				$ 0.01		$ 0.01
Shares converted (shares)			140
Preferred stock, depositary shares (shares)						13,749,860
Conversion factor, depository shares, preferred stock			0.8687
Conversion of preferred stock (shares)						0
Subsequent Event | Preferred stock | Convertible Preferred Stock Subject to Mandatory Redemption | Series B Preferred Stock
Class of Stock [Line Items]
Conversion factor, preferred stock (shares)	8.7420
Dividends	$ 18,900,000
Conversion factor, depository shares, preferred stock	0.8742
Conversion of preferred stock (shares)	12,020,064
2011 Buyback
Class of Stock [Line Items]
Authorized repurchase of common stock									$ 1,500,000,000
Treasury stock activity (in shares)						6,099,150
Treasury stock activity						$ 766,300,000
Treasury stock, shares						12,356,054
Treasury stock						$ 1,200,000,000
2017 Buyback
Class of Stock [Line Items]
Authorized repurchase of common stock						2,000,000,000
Treasury stock activity						$ 0

Equity (Distributions) (Detail)

Equity (Distributions) (Detail) - USD ($) $ / shares in Units, $ in Millions	Dec. 14, 2016	Dec. 02, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Dividends Payable [Line Items]
Payments of ordinary dividends, common stock			$ 1,073	$ 886.1	$ 710.9
Preferred stock dividends declared			$ 91.4	$ 107.1	$ 76.8
Common Stock
Dividends Payable [Line Items]
Common stock, dividends declared per share (in dollars per share)	$ 0.70		$ 2.6200	$ 2.1700	$ 1.8100
Dividends declared, common stock, percent of total			100.00%	100.00%	100.00%
Common Stock | Ordinary Income
Dividends Payable [Line Items]
Common stock, dividends declared per share (in dollars per share)			$ 2.6200	$ 2.1700	$ 1.2694
Dividends declared, common stock, percent of total			100.00%	100.00%	70.13%
Common Stock | Capital Gain
Dividends Payable [Line Items]
Common stock, dividends declared per share (in dollars per share)			$ 0	$ 0	$ 0.5406
Dividends declared, common stock, percent of total			0.00%	0.00%	29.87%
Common Stock | Dividends Declared and Paid
Dividends Payable [Line Items]
Common stock, dividends declared per share (in dollars per share)			$ 2.62	$ 2.17	$ 1.81
Payments of ordinary dividends, common stock			$ 1,122.5	$ 923.7	$ 766.4
Series A Preferred Stock | Convertible Preferred Stock Subject to Mandatory Redemption
Dividends Payable [Line Items]
Preferred stock, dividends per share, declared (in dollars per share)		$ 1.3125	$ 3.3643	$ 6.4578	$ 5.2500
Dividends declared, preferred stock, declared			100.00%	100.00%	100.00%
Series A Preferred Stock | Convertible Preferred Stock Subject to Mandatory Redemption | Ordinary Income
Dividends Payable [Line Items]
Preferred stock, dividends per share, declared (in dollars per share)			$ 3.3643	$ 6.4578	$ 3.6818
Dividends declared, preferred stock, declared			100.00%	100.00%	70.13%
Series A Preferred Stock | Convertible Preferred Stock Subject to Mandatory Redemption | Capital Gain
Dividends Payable [Line Items]
Preferred stock, dividends per share, declared (in dollars per share)			$ 0	$ 0	$ 1.5682
Dividends declared, preferred stock, declared			0.00%	0.00%	29.87%
Series A Preferred Stock | Dividends Declared and Paid | Convertible Preferred Stock Subject to Mandatory Redemption
Dividends Payable [Line Items]
Preferred stock, dividends per share, declared (in dollars per share)			$ 2.63	$ 5.25	$ 3.94
Preferred stock dividends declared			$ 15.8	$ 31.5	$ 23.7
Series B Preferred Stock | Convertible Preferred Stock Subject to Mandatory Redemption
Dividends Payable [Line Items]
Preferred stock, dividends per share, declared (in dollars per share)			$ 6.5233	$ 5.5000	$ 3.8653
Dividends declared, preferred stock, declared			100.00%	100.00%	100.00%
Series B Preferred Stock | Convertible Preferred Stock Subject to Mandatory Redemption | Ordinary Income
Dividends Payable [Line Items]
Preferred stock, dividends per share, declared (in dollars per share)			$ 6.5233	$ 5.5000	$ 2.7107
Dividends declared, preferred stock, declared			100.00%	100.00%	70.13%
Series B Preferred Stock | Convertible Preferred Stock Subject to Mandatory Redemption | Capital Gain
Dividends Payable [Line Items]
Preferred stock, dividends per share, declared (in dollars per share)			$ 0	$ 0	$ 1.1546
Dividends declared, preferred stock, declared			0.00%	0.00%	29.87%
Series B Preferred Stock | Dividends Declared and Paid | Convertible Preferred Stock Subject to Mandatory Redemption
Dividends Payable [Line Items]
Preferred stock, dividends per share, declared (in dollars per share)			$ 55	$ 55	$ 38.65
Preferred stock dividends declared			$ 75.6	$ 75.6	$ 53.1

IMPAIRMENTS, NET LOSS ON SAL103

IMPAIRMENTS, NET LOSS ON SALE OF LONG-LIVED ASSETS (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Finite-Lived Intangible Assets [Line Items]
Impairment charges and net losses on sales or disposals of long-lived assets	$ 244.2	$ 53.6	$ 29.8
Impairment charges	211.4	28.5	15.1
Losses associated with the sale or disposal of certain non-core towers, other assets and other miscellaneous items	32.8	25.1	$ 14.7
Current net book value of intangibles	11,783.3	11,274.6
Network location intangible assets
Finite-Lived Intangible Assets [Line Items]
Impairment charges	81
Current net book value of intangibles	3,333.5	3,342
Tenant-related intangible assets
Finite-Lived Intangible Assets [Line Items]
Impairment charges	100.1
Current net book value of intangibles	8,396.2	$ 7,906.4
Tata Teleservices Limited | Tenant-related intangible assets
Finite-Lived Intangible Assets [Line Items]
Current net book value of intangibles	$ 436.4

EARNINGS PER COMMON SHARE (Sche

EARNINGS PER COMMON SHARE (Schedule of Earnings Per Basic And Diluted by Common Class) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income attributable to American Tower Corporation stockholders	$ 238.5	$ 317.3	$ 367	$ 316.1	$ 229.2	$ 264.5	$ 187.6	$ 275.2	$ 1,238.9	$ 956.4	$ 685.1
Dividends on preferred stock	(18.9)	(18.9)	(22.8)	(26.8)	(26.8)	(26.8)	(26.8)	(26.8)	(87.4)	(107.1)	(90.2)
NET INCOME ATTRIBUTABLE TO AMERICAN TOWER CORPORATION COMMON STOCKHOLDERS	$ 219.6	$ 298.4	$ 344.2	$ 289.3	$ 202.4	$ 237.7	$ 160.8	$ 248.4	$ 1,151.5	$ 849.3	$ 594.9
Basic weighted average common shares outstanding									428,181	425,143	418,907
Dilutive securities (shares)									3,507	4,140	4,108
Diluted weighted average common shares outstanding									431,688	429,283	423,015
Basic net income attributable to American Tower Corporation common stockholders (in dollars per share)	$ 0.51	$ 0.70	$ 0.81	$ 0.68	$ 0.48	$ 0.56	$ 0.38	$ 0.59	$ 2.69	$ 2	$ 1.42
Diluted net income attributable to American Tower Corporation common stockholders (in dollars per share)	$ 0.51	$ 0.69	$ 0.80	$ 0.67	$ 0.47	$ 0.55	$ 0.37	$ 0.58	$ 2.67	$ 1.98	$ 1.41

EARNINGS PER COMMON SHARE (S105

EARNINGS PER COMMON SHARE (Schedule of Shares Excluded From Computation of Earnings Per Share) (Details) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restricted stock awards
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Shares issuable upon conversion of the stock-based awards and convertible notes	3	6	0
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Shares issuable upon conversion of the stock-based awards and convertible notes	4	817	1,606
Preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Shares issuable upon conversion of the stock-based awards and convertible notes	14,040	17,509	15,408

COMMITMENTS AND CONTINGENCIE106

COMMITMENTS AND CONTINGENCIES (Narrative) (Details) ₨ in Millions	Dec. 08, 2016tower	Dec. 05, 2016USD ($)	Dec. 05, 2016INR (₨)	Mar. 27, 2015USD ($)communications_siterenewal_period	Dec. 31, 2000tower	Dec. 31, 2017USD ($)towerrenewal_periodshares	Dec. 31, 2016USD ($)tower	Dec. 31, 2015USD ($)
Loss Contingencies [Line Items]
Loss contingency, foreign income tax assessment		$ 69,800,000	₨ 4,750
Aggregate rent expense | $						$ 1,088,000,000	$ 986,200,000	$ 804,800,000
Initial lease term						10 years
Verizon Transaction
Loss Contingencies [Line Items]
Capital leased assets, number of units | communications_site				11,300
Right to lease, weighted average term				28 years
Aggregate purchase option price for towers | $				$ 5,000,000,000
Customer lease, initial term				10 years
Successive terms to renew lease | renewal_period				8
Renewal term				5 years
At T Transaction
Loss Contingencies [Line Items]
Capital leased assets, number of units | tower						2,350
Aggregate purchase option price for towers | $						$ 831,300,000
Successive terms to renew lease | renewal_period						4
Renewal term						5 years
Operating lease, term of contract						27 years
Number of communications sites acquired | tower						88
Purchase price accretion rate (per year)						10.00%
At T Transaction | Commitments subsequent to June 30, 2020
Loss Contingencies [Line Items]
Renewal term						1 year
ALLTEL Transaction
Loss Contingencies [Line Items]
Capital leased assets, number of units | tower					1,800
Aggregate purchase option price for towers | $						$ 10,400,000
Average lease term (in years)					15 years
Number of units acquired through purchase option | tower	1,523
Number of unit with intent to exercise purchase option | tower							243
Cash purchase price per tower | $							$ 42,844
Purchase price of tower in shares of common stock | shares						769

COMMITMENTS AND CONTINGENCIE107

COMMITMENTS AND CONTINGENCIES (Future Minimum Rental Payments Under Non-Cancelable Operating Leases) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2,018	$ 924
2,019	887
2,020	848
2,021	811
2,022	768
Thereafter	$ 6,533
Total		$ 10,771

COMMITMENTS AND CONTINGENCIE108

COMMITMENTS AND CONTINGENCIES (Future Minimum Payments Under Capital Leases) (Details) $ in Millions	Dec. 31, 2017USD ($)
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2,018	$ 34
2,019	31
2,020	26
2,021	21
2,022	18
Thereafter	166
Total minimum lease payments	296
Less amounts representing interest	(130)
Present value of capital lease obligations	$ 166

COMMITMENTS AND CONTINGENCIE109

COMMITMENTS AND CONTINGENCIES (Future Minimum Rental Receipts Under Operating Lease Agreements) (Details) $ in Millions	Dec. 31, 2017USD ($)
Operating Leases, Future Minimum Payments Receivable [Abstract]
2,018	$ 4,927
2,019	4,683
2,020	4,393
2,021	3,957
2,022	3,095
Thereafter	11,180
Total	$ 32,235

SUPPLEMENTAL CASH FLOW INFOR110

SUPPLEMENTAL CASH FLOW INFORMATION (Supplemental Cash Flow Information and Non-Cash Investing and Financing Activities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental cash flow information:
Cash paid for interest	$ 712.1	$ 645.1	$ 578
Cash paid for income taxes (net of refunds of $20.7, $19.6 and $7.1, respectively)	136.5	96.2	157.1
Non-cash investing and financing activities:
Increase (decrease) in accounts payable and accrued expenses for purchases of property and equipment and construction activities	34	(19)	2.8
Purchases of property and equipment under capital leases	54.8	55.6	36.9
Fair value of debt assumed through acquisitions	0	786.9	0
Exercise of purchase option for property and equipment for common shares issued	0	120.8	0
Settlement of accounts receivable related to acquisitions	0	0	0.9
Conversion of third-party debt to equity	48.2	0	0
Proceeds from income tax refunds	$ 20.7	$ 19.6	$ 7.1

BUSINESS SEGMENTS (Summarized F

BUSINESS SEGMENTS (Summarized Financial Information Concerning the Company's Reportable Segments) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Segment revenues	$ 1,704.5	$ 1,680.7	$ 1,662.5	$ 1,616.2	$ 1,539.5	$ 1,514.8	$ 1,442.2	$ 1,289	$ 6,663.9	$ 5,785.7	$ 4,771.5
Segment operating expenses									2,053.7	1,788	1,306.8
Segment gross margin									4,621	4,008.6	3,475.9
Segment selling, general, administrative and development expense									392.9	329.9	287.9
Segment operating profit									4,228.1	3,678.7	3,188
Stock-based compensation expense									108.5	89.9	90.5
Other selling, general, administrative and development expense									138.5	126	121.4
Depreciation, amortization and accretion									1,715.9	1,525.6	1,285.3
Other expense									1,009.1	811.3	860.8
INCOME FROM CONTINUING OPERATIONS BEFORE INCOME TAXES									1,256.1	1,125.9	830
Capital expenditures									835.4	701.4	728.8
Repayment of capital lease for capital expenditures									31.8	18.9
Operating Expense
Segment Reporting Information [Line Items]
Stock-based compensation expense									2.9	2.4	2
Selling General Administrative And Development Expense
Segment Reporting Information [Line Items]
Stock-based compensation expense									105.6	87.5	88.5
TV Azteca
Segment Reporting Information [Line Items]
Interest income, TV Azteca, net of interest expense of $1.2, $1.2 and $0.8, respectively									10.8	10.9	11.2
U.S.
Segment Reporting Information [Line Items]
Segment revenues									3,703.7	3,442.7	3,248.6
Other
Segment Reporting Information [Line Items]
Stock-based compensation expense									108.5	89.9
Other selling, general, administrative and development expense									138.5	126
Depreciation, amortization and accretion									1,715.9	1,525.6
Other expense									1,009.1	811.3
Capital expenditures									17.7
Property | Operating Segments
Segment Reporting Information [Line Items]
Segment revenues									6,565.9	5,713.1	4,680.4
Segment operating expenses									2,019.9	1,761	1,273.8
Segment gross margin									4,556.8	3,963	3,417.8
Segment selling, general, administrative and development expense									379.2	317.4	272.2
Segment operating profit									4,177.6	3,645.6	3,145.6
Capital expenditures									817.7	684.9	711.5
Property | Operating Segments | TV Azteca
Segment Reporting Information [Line Items]
Interest income, TV Azteca, net of interest expense of $1.2, $1.2 and $0.8, respectively									10.8	10.9	11.2
Property | Operating Segments | U.S.
Segment Reporting Information [Line Items]
Segment revenues									3,605.7	3,370.1	3,157.5
Segment operating expenses									746.5	733.4	678.5
Segment gross margin									2,859.2	2,636.7	2,479
Segment selling, general, administrative and development expense									151.4	147.6	138.6
Segment operating profit									2,707.8	2,489.1	2,340.4
Capital expenditures									360.6	310.7	367.7
Property | Operating Segments | U.S. | TV Azteca
Segment Reporting Information [Line Items]
Interest income, TV Azteca, net of interest expense of $1.2, $1.2 and $0.8, respectively									0	0	0
Property | Operating Segments | Asia
Segment Reporting Information [Line Items]
Segment revenues									1,164.4	827.6	242.2
Segment operating expenses									649	465.9	126.9
Segment gross margin									515.4	361.7	115.3
Segment selling, general, administrative and development expense									82.4	48.2	22.7
Segment operating profit									433	313.5	92.6
Capital expenditures									118	115.5	75.4
Property | Operating Segments | Asia | TV Azteca
Segment Reporting Information [Line Items]
Interest income, TV Azteca, net of interest expense of $1.2, $1.2 and $0.8, respectively									0	0	0
Property | Operating Segments | EMEA
Segment Reporting Information [Line Items]
Segment revenues									626.2	529.5	395.1
Segment operating expenses									238.3	223.7	163.8
Segment gross margin									387.9	305.8	231.3
Segment selling, general, administrative and development expense									67.9	60.9	48.7
Segment operating profit									320	244.9	182.6
Capital expenditures									141.7	86.1	66.6
Property | Operating Segments | EMEA | TV Azteca
Segment Reporting Information [Line Items]
Interest income, TV Azteca, net of interest expense of $1.2, $1.2 and $0.8, respectively									0	0	0
Property | Operating Segments | Latin America
Segment Reporting Information [Line Items]
Segment revenues									1,169.6	985.9	885.6
Segment operating expenses									386.1	338	304.6
Segment gross margin									794.3	658.8	592.2
Segment selling, general, administrative and development expense									77.5	60.7	62.2
Segment operating profit									716.8	598.1	530
Capital expenditures									197.4	172.6	201.8
Property | Operating Segments | Latin America | TV Azteca
Segment Reporting Information [Line Items]
Interest income, TV Azteca, net of interest expense of $1.2, $1.2 and $0.8, respectively									10.8	10.9	11.2
Services | Operating Segments
Segment Reporting Information [Line Items]
Segment revenues									98	72.6	91.1
Segment operating expenses									33.8	27	33
Segment gross margin									64.2	45.6	58.1
Segment selling, general, administrative and development expense									13.7	12.5	15.7
Segment operating profit									50.5	33.1	42.4
Capital expenditures									0	0	0
Services | Operating Segments | TV Azteca
Segment Reporting Information [Line Items]
Interest income, TV Azteca, net of interest expense of $1.2, $1.2 and $0.8, respectively									$ 0	0	0
Other
Segment Reporting Information [Line Items]
Stock-based compensation expense											90.5
Other selling, general, administrative and development expense											121.4
Depreciation, amortization and accretion											1,285.3
Other expense											860.8
Capital expenditures										$ 16.5	$ 17.3

BUSINESS SEGMENTS (Additional I

BUSINESS SEGMENTS (Additional Information Relating to the Company's Operating Segments) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	$ 33,214.3	$ 30,879.2	$ 26,904.3
Corporate, Non-Segment
Segment Reporting, Asset Reconciling Item [Line Items]
Assets			162.4
Operating Segments | Property | U.S.
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	19,032.6	18,846.9	19,286.5
Operating Segments | Property | Asia
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	4,770.8	4,535.3	736.1
Operating Segments | Property | EMEA
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	3,213.6	2,062.4	2,249.6
Operating Segments | Property | Latin America
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	5,868.4	4,938.1	4,401.3
Operating Segments | Services
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	42.3	48.3	$ 68.4
Corporate, Non-Segment
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	$ 286.6	$ 448.2

BUSINESS SEGMENTS (Summarized G

BUSINESS SEGMENTS (Summarized Geographic Information Related to the Company's Operating Revenues and Long-Lived Assets) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment revenues	$ 1,704.5	$ 1,680.7	$ 1,662.5	$ 1,616.2	$ 1,539.5	$ 1,514.8	$ 1,442.2	$ 1,289	$ 6,663.9	$ 5,785.7	$ 4,771.5
Long-Lived Assets	28,522.7				26,862.6				28,522.7	26,862.6
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment revenues									3,703.7	3,442.7	3,248.6
Long-Lived Assets	16,930.2				16,969.6				16,930.2	16,969.6
India
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment revenues									1,164.4	827.6	242.2
Long-Lived Assets	4,052.6				4,094.2				4,052.6	4,094.2
France
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment revenues									59.5	0	0
Long-Lived Assets	1,009.6				0				1,009.6	0
Germany
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment revenues									63.1	60.2	56
Long-Lived Assets	428				397.3				428	397.3
Ghana
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment revenues									122.9	116.2	94.5
Long-Lived Assets	171.4				192.2				171.4	192.2
Nigeria
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment revenues									213.9	215.4	109.7
Long-Lived Assets	587.2				640.6				587.2	640.6
South Africa
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment revenues									106.5	80	80.5
Long-Lived Assets	330.4				271.8				330.4	271.8
Uganda
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment revenues									60.3	57.7	54.4
Long-Lived Assets	136.9				141.5				136.9	141.5
Argentina
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment revenues									15.9	1	0
Long-Lived Assets	117.9				137.6				117.9	137.6
Brazil
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment revenues									620.1	506.2	408.6
Long-Lived Assets	2,557.4				2,626.4				2,557.4	2,626.4
Chile
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment revenues									40.4	33.8	29.7
Long-Lived Assets	151.2				137.2				151.2	137.2
Colombia
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment revenues									89.3	79.7	78.4
Long-Lived Assets	369				272.3				369	272.3
Costa Rica
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment revenues									19.4	19	17.2
Long-Lived Assets	112.9				117.5				112.9	117.5
Mexico
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment revenues									364.3	331.2	340.5
Long-Lived Assets	1,396.8				797.8				1,396.8	797.8
Paraguay
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment revenues									2.7	0	0
Long-Lived Assets	77.5				0				77.5	0
Peru
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment revenues									17.5	15	11.2
Long-Lived Assets	93.7				66.6				93.7	66.6
Total International
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment revenues									2,960.2	2,343	$ 1,522.9
Long-Lived Assets	$ 11,592.5				$ 9,893				$ 11,592.5	$ 9,893

BUSINESS SEGMENTS (Major Custom

BUSINESS SEGMENTS (Major Customers) (Details) - Customer Concentration Risk - Sales Revenue, Net	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue, Major Customer [Line Items]
Concentration risk, percentage	10.00%	10.00%	10.00%
AT&T
Revenue, Major Customer [Line Items]
Concentration risk, percentage	19.00%	21.00%	24.00%
Verizon Wireless
Revenue, Major Customer [Line Items]
Concentration risk, percentage	16.00%	15.00%	16.00%
Sprint
Revenue, Major Customer [Line Items]
Concentration risk, percentage	9.00%	11.00%	13.00%
T-Mobile
Revenue, Major Customer [Line Items]
Concentration risk, percentage	9.00%	9.00%	10.00%

SELECTED QUARTERLY FINANCIAL115

SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Selected Quarterly Financial Information [Abstract]
Operating revenues	$ 1,704.5	$ 1,680.7	$ 1,662.5	$ 1,616.2	$ 1,539.5	$ 1,514.8	$ 1,442.2	$ 1,289	$ 6,663.9	$ 5,785.7	$ 4,771.5
Cost of operations	526.9	519.8	517.2	492.7	488	491.2	459.7	351.4	2,056.6	1,790.4
Operating income	329	561.1	576.9	531.4	489.3	479.1	432.8	451.9	1,998.4	1,853	1,612.8
Net income	194.8	334.7	388.5	307.4	232.9	263.7	192.5	281.3	1,225.4	970.4	672
Net income attributable to American Tower Corporation stockholders	238.5	317.3	367	316.1	229.2	264.5	187.6	275.2	1,238.9	956.4	685.1
Dividends on preferred stock	(18.9)	(18.9)	(22.8)	(26.8)	(26.8)	(26.8)	(26.8)	(26.8)	(87.4)	(107.1)	(90.2)
NET INCOME ATTRIBUTABLE TO AMERICAN TOWER CORPORATION COMMON STOCKHOLDERS	$ 219.6	$ 298.4	$ 344.2	$ 289.3	$ 202.4	$ 237.7	$ 160.8	$ 248.4	$ 1,151.5	$ 849.3	$ 594.9
Basic net income per common share (in dollars per share)	$ 0.51	$ 0.70	$ 0.81	$ 0.68	$ 0.48	$ 0.56	$ 0.38	$ 0.59	$ 2.69	$ 2	$ 1.42
Diluted net income per common share (in dollars per share)	$ 0.51	$ 0.69	$ 0.80	$ 0.67	$ 0.47	$ 0.55	$ 0.37	$ 0.58	$ 2.67	$ 1.98	$ 1.41

Schedule III - SCHEDULE OF R116

Schedule III - SCHEDULE OF REAL ESTATE AND ACCUMULATED DEPRECIATION (Schedule Of Real Estate And Accumulated Depreciation) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)site	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Number of units | site	149,246
Encumbrances	$ 3,435.3
Gross amount carried at close of current period	15,349	$ 14,277	$ 13,046.3	$ 10,434.3
Accumulated depreciation at close of current period	$ (5,181.2)	$ (4,548.1)	$ (3,994.9)	$ (3,613.1)
Date of construction	Various
Date acquired	Various
Percentage exceeds total amounts	5.00%
Maximum
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Life on which depreciation in latest income statements is computed	20 years

Schedule III - SCHEDULE OF R117

Schedule III - SCHEDULE OF REAL ESTATE AND ACCUMULATED DEPRECIATION (Activity Of Real Estate And Accumulated Depreciation) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
SEC Schedule III, Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Gross amount at beginning	$ 14,277	$ 13,046.3	$ 10,434.3
Acquisitions	499.7	787.2	2,620.8
Discretionary capital projects	120.7	105.3	210.4
Discretionary ground lease purchases	150.4	168.1	144.7
Redevelopment capital expenditures	138.8	136.8	114.1
Capital improvements	65.6	81.8	42.4
Start-up capital expenditures	158.1	128.7	35.6
Other additions	106.4	139.4	201.1
Total additions	1,239.7	1,547.3	3,369.1
Cost of real estate sold or disposed	(246.5)	(85.8)	(61)
Other deductions	78.8	(230.8)	(696.1)
Total deductions	(167.7)	(316.6)	(757.1)
Balance at end	15,349	14,277	13,046.3
SEC Schedule III, Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Gross amount of accumulated depreciation at beginning	(4,548.1)	(3,994.9)	(3,613.1)
Depreciation	(718.7)	(647.9)	(557.1)
Other additions	0	0	0
Total additions	(718.7)	(647.9)	(557.1)
Amount of accumulated depreciation for assets sold or disposed	100.7	24.9	30.1
Other deductions	(15.1)	69.8	145.2
Total deductions	85.6	94.7	175.3
Balance at end	$ (5,181.2)	$ (4,548.1)	$ (3,994.9)

OTHER NON-CURRENT LIABILITIE118

OTHER NON-CURRENT LIABILITIES (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Other Liabilities, Noncurrent [Abstract]
Unearned revenue	$ 509.2	$ 457.3
Deferred rent liability	467	407.2
Other miscellaneous liabilities	268	278.1
Other non-current liabilities	$ 1,244.2	$ 1,142.6