4. Intangible Assets and Liabilities

The net carrying value of the Company's goodwill and identifiable intangible assets consist of the following:

	December 31,
	2014		2015
	(in thousands)
Goodwill	$	1,642,083	$	2,314,624
Identifiable intangibles—Indefinite lived assets:
Trademarks		57,709		162,609
Certificates of need		12,727		13,022
Accreditations		2,083		2,045
Identifiable intangibles—Finite lived assets:
Customer relationships		—		132,751
Favorable leasehold interests		—		8,248

Total identifiable intangibles	$	1,714,602	$	2,633,299

The Company's customer relationship assets amortize over their estimated useful lives. Amortization expense for the Company's customer relationships was $8.5 million for the year ended December 31, 2015. Estimated amortization expense of the Company's customer relationships for each of the five succeeding years is $14.6 million.

In addition, the Company has recognized unfavorable leasehold interests which are recorded as liabilities. The net carrying value of unfavorable leasehold interests was $3.0 million as of December 31, 2015.

The Company's favorable leasehold assets and unfavorable leasehold liabilities are amortized to rent expense over the remaining term of their respective leases to reflect a market rent per period based upon the market conditions present at the acquisition date. The effect of this amortization increased rent expense by $0.3 million for the year ended December 31, 2015.

The Company's accreditations and trademarks have renewal terms. The costs to renew these intangibles are expensed as incurred. At December 31, 2015, the accreditations and trademarks have a weighted average time until next renewal of 1.5 years and 3.8 years, respectively.

The changes in the carrying amount of goodwill for the Company's reportable segments for the years ended December 31, 2014 and 2015 are as follows:

	Specialty Hospitals			Outpatient Rehabilitation			Concentra			Total
	(in thousands)
Balance as of January 1, 2014	$	1,334,615		$	308,018					$	1,642,633
Goodwill acquired during year		855			1,011						1,866
Goodwill allocated to contributed business		—			(2,406	)					(2,406	)
Purchase accounting adjustment		(10	)		—						(10	)

Balance as of December 31, 2014	$	1,335,460		$	306,623		$	—		$	1,642,083
Goodwill acquired during year		21,972			—			702,023			723,995
Measurement period adjustment		(53	)		—			(51,373	)		(51,426	)
Disposal of business		—			(28	)		—			(28	)

Balance as of December 31, 2015	$	1,357,379		$	306,595		$	650,650		$	2,314,624

See Note 2 for details of the goodwill acquired during the period.