Intangible Assets and Liabilities

The gross carrying amount and accumulated amortization (accretion) of our intangible assets and liabilities, other than goodwill, were as follows:



	March 31, 2019									December 31, 2018
In thousands	Gross			Accumulated (Amortization) Accretion			Net			Gross			Accumulated (Amortization) Accretion			Net
Intangible Assets
Core-developed technology	$	507,023		$	(436,829	)	$	70,194		$	507,100		$	(429,955	)	$	77,145
Customer contracts and relationships	382,504			(224,312		)	158,192			379,614			(212,538		)	167,076
Trademarks and trade names	79,071			(71,145		)	7,926			78,746			(69,879		)	8,867
Other	12,021			(11,245		)	776			12,600			(11,205		)	1,395
Total intangible assets subject to amortization	980,619			(743,531		)	237,088			978,060			(723,577		)	254,483
In-process research and development	2,900			—			2,900			3,100			—			3,100
Total intangible assets	$	983,519		$	(743,531	)	$	239,988		$	981,160		$	(723,577	)	$	257,583

Intangible Liabilities
Customer contracts and relationships	$	(23,900	)	$	7,275		$	(16,625	)	$	(23,900	)	$	5,217		$	(18,683	)

A summary of intangible assets and liabilities activity is as follows:



	Three Months Ended March 31,
In thousands	2019			2018
Beginning balance, intangible assets, gross	$	981,160		$	769,851
Intangible assets acquired	—			240,600
Effect of change in exchange rates	2,359			22,979
Ending balance, intangible assets, gross	$	983,519		$	1,033,430

Beginning balance, intangible liabilities, gross	$	(23,900	)	$	—
Intangible liabilities acquired	—			(23,900		)
Effect of change in exchange rates	—			—
Ending balance, intangible liabilities, gross	$	(23,900	)	$	(23,900	)

On January 5, 2018, we completed our acquisition of Silver Spring Networks, Inc. (SSNI) by purchasing 100% of the voting stock. Acquired intangible assets include in-process research and development (IPR&D), which is not amortized until such time as the associated development projects are completed. Of these projects, $0.2 million were completed during the three months ended March 31, 2019 and are included in core-developed technology. The remaining IPR&D is expected to be completed in 2019. Acquired intangible liabilities reflect the present value of the projected cash outflows for an existing contract where remaining costs are expected to exceed projected revenues.

Estimated future annual amortization (accretion) is as follows:



Year Ending December 31,	Amortization		Accretion			Estimated Annual Amortization, net
In thousands
2019 (amount remaining at March 31, 2019)	$	54,492	$	(6,175	)	$	48,317
2020	52,891		(8,028		)	44,863
2021	37,209		(1,963		)	35,246
2022	27,015		(459		)	26,556
2023	19,366		—			19,366
Beyond 2023	46,115		—			46,115
Total intangible assets subject to amortization (accretion)	$	237,088	$	(16,625	)	$	220,463

We have recognized $16.0 million and $17.7 million of net amortization of intangible assets for the three months ended March 31, 2019 and 2018, respectively, within operating expenses in the Consolidated Statement of Operations. These expenses relate to intangible assets and liabilities acquired as part of business combinations.