| Intangible Assets and Deferred Credits |
Intangible Assets Goodwill The changes in the carrying value of goodwill for the year ended December 31, 2013 were as follows: | | | | | Balance as of December 31, 2011 | $ | 2,035,390 |
| Additions due to NextG Acquisition(a) | 573,617 |
| Additions due to WCP Acquisition (b) | 54,824 |
| Additions due to T-Mobile Acquisition (c) | 428,019 |
| Additions due to other acquisitions | 28,113 |
| Effect of exchange rate fluctuations | (6 | ) | Balance as of December 31, 2012 | $ | 3,119,957 |
| Additions due to AT&T Acquisition (c) | 1,768,535 |
| Additions due to other acquisitions | 25,194 |
| Effect of exchange rate fluctuations and other adjustments | 2,740 |
| Balance as of December 31, 2013 | $ | 4,916,426 |
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| | (a) | The purchase price allocation for the NextG Acquisition resulted in the recognition of a substantial amount of goodwill at CCUSA relative to the purchase price based on the following: |
| | • | the acquired or in-process DAS have low average tenancy, which the Company believes provides an opportunity to co-locate additional tenants on those systems; |
| | • | the Company believes that the economics associated with DAS are similar to the economics associated with the Company's towers, whereby expected increases in revenues from additional tenants on existing DAS are expected to result in high incremental margins due to relatively fixed operating costs; |
| | • | the Company believes the demand for tenants to co-locate on DAS will be driven by the continued growth trends in the wireless communication industry as wireless carriers continue to focus on improving network quality and expanding capacity; |
| | • | the Company believes the acquired DAS are well-positioned to benefit from the anticipated growth in the wireless industry with their previously mentioned locations in the ten largest metropolitan statistical areas in the U.S.; or |
| | • | other intangibles not qualified for separate recognition, including the assembled work force. |
To a lesser extent, a portion of the goodwill recognized is the result of recording the tax impact of the NextG Acquisition. See also note 10. | | (b) | The Company paid a purchase price for the WCP Acquisition that resulted in goodwill at CCUSA primarily because of the strategic opportunities related to the acquired portfolio. |
| | (c) | The purchase price allocation for the T-Mobile Acquisition and the preliminary purchase price allocation for the AT&T Acquisition resulted in the recognition of goodwill at CCUSA primarily because of the anticipated growth opportunities in the respective tower portfolios. |
Intangibles The following is a summary of the Company's intangible assets. See note 3 for further discussion of the Company's acquisitions. | | | | | | | | | | | | | | | | | | | | | | | | | | As of December 31, 2013 | | As of December 31, 2012 | | Gross Carrying Value | | Accumulated Amortization | | Net Book Value | | Gross Carrying Value | | Accumulated Amortization | | Net Book Value | Site rental contracts and customer relationships | $ | 4,761,605 |
| | $ | (1,111,262 | ) | | $ | 3,650,343 |
| | $ | 3,566,207 |
| | $ | (913,647 | ) | | $ | 2,652,560 |
| Other intangible assets | 486,751 |
| | (79,229 | ) | | 407,522 |
| | 354,208 |
| | (65,072 | ) | | 289,136 |
| Total | $ | 5,248,356 |
| | $ | (1,190,491 | ) | | $ | 4,057,865 |
| | $ | 3,920,415 |
| | $ | (978,719 | ) | | $ | 2,941,696 |
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The components of the additions to intangible assets during the years ended December 31, 2013 and 2012 are as follows: | | | | | | | | | | | | | | For Years Ended December 31, | | 2013 | | 2012 | | Amount | | Weighted-Average Amortization Period | | Amount | | Weighted-Average Amortization Period | | | | (In years) | | | | (In years) | Site rental contracts and customer relationships | $ | 1,203,596 |
| | 20 | | $ | 741,526 |
| | 21 | Other intangible assets | 132,763 |
| (a) | 21 | | 208,700 |
| (b) | 19 | Total | $ | 1,336,359 |
| | 20 | | $ | 950,226 |
| | 21 |
| | (a) | $114.9 million is related to below-market leases for land interests under acquired wireless infrastructure for the year ended December 31, 2013 which is inclusive of adjustments made during 2013 related to the T-Mobile purchase price allocation (see note 3). |
| | (b) | $178.3 million is related to below-market leases for land interests under acquired wireless infrastructure for the year ended December 31, 2012. |
Amortization expense related to intangible assets is classified as follows on the Company's consolidated statement of operations and comprehensive income (loss): | | | | | | | | | | | | | | For Years Ended December 31, | Classification | 2013 | | 2012 | | 2011 | Depreciation, amortization and accretion | $ | 204,042 |
| | $ | 177,163 |
| | $ | 159,478 |
| Site rental costs of operations | 10,197 |
| | 3,352 |
| | 3,709 |
| Total amortization expense | $ | 214,239 |
| | $ | 180,515 |
| | $ | 163,187 |
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The estimated annual amortization expense related to intangible assets (inclusive of those recorded as an increase to "site rental costs of operations") for the years ended December 31, 2014 to 2018 is as follows: | | | | | | | | | | | | | | | | | | | | | | Years Ending December 31, | | 2014 |
| 2015 |
| 2016 |
| 2017 |
| 2018 | Estimated annual amortization | $ | 272,944 |
| | $ | 267,326 |
| | $ | 267,273 |
| | $ | 266,902 |
| | $ | 266,732 |
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