4. Acquired Intangible Assets and Liabilities

The following summarizes our acquired intangible assets (acquired in place lease value and acquired above-market lease value) and intangible liabilities (acquired below-market lease value) as of March 31, 2012 and December 31, 2011.


	Balance as of
(Amounts in thousands)	March 31, 2012			December 31, 2011
Acquired in place lease value:
Gross amount	$	569,669		$	545,409
Accumulated amortization		(325,041	)		(312,499	)

Net	$	244,628		$	232,910

Acquired above-market lease value:
Gross amount	$	87,980		$	87,800
Accumulated amortization		(60,385	)		(58,099	)

Net	$	27,595		$	29,701

Acquired below-market lease value:
Gross amount	$	228,490		$	201,275
Accumulated amortization		(120,220	)		(115,456	)

Net	$	108,270		$	85,819

Amortization of acquired below-market lease value, net of acquired above-market lease value, resulted in an increase to rental revenues of $2.2 million and $1.8 million for the three months ended March 31, 2012 and 2011, respectively. The expected average remaining lives for acquired below market leases and acquired above market leases is 6.8 years and 4.4 years, respectively, as of March 31, 2012. Estimated annual amortization of acquired below-market lease value, net of acquired above-market lease value, for each of the five succeeding years, commencing January 1, 2013 is as follows:


(Amounts in thousands)
2013	$	11,684
2014		9,852
2015		8,968
2016		8,088
2017		7,160

Costs associated with extending or renewing acquired leases are capitalized and classified as deferred leasing cost. Amortization of acquired in place lease value (a component of depreciation and amortization expense) was $12.0 million and $14.3 million for the three months ended March 31, 2012 and 2011, respectively. The expected average amortization period for acquired in place lease value is 6.2 years as of March 31, 2012. The weighted average remaining contractual life for acquired leases excluding renewals or extensions is 5.3 years as of March 31, 2012. Estimated annual amortization of acquired in place lease value for each of the five succeeding years, commencing January 1, 2013 is as follows:


(Amounts in thousands)
2013	$	44,791
2014		39,594
2015		31,370
2016		29,134
2017		14,858