GOODWILL AND OTHER INTANGIBLE ASSETS

Changes in the carrying amount of goodwill consists of the following activity for the three and nine months ended September 30, 2019 (in thousands):



Three Months Ended September 30, 2019
June 30, 2019 - net book value	$	632,469
Acquisitions	786,904
September 30, 2019 - net book value	$	1,419,373

Nine Months Ended September 30, 2019
December 31, 2018 - net book value	$	632,469
Acquisitions	786,904
September 30, 2019 - net book value	$	1,419,373

Changes in the carrying amount of goodwill consists of the following activity for the three and nine months ended September 30, 2018 (in thousands):



Three Months Ended September 30, 2018
June 30, 2018 - net book value	$	627,392
Acquisitions	4,492
September 30, 2018 - net book value	$	631,884

Nine Months Ended September 30, 2018
December 31, 2017 - net book value	$	627,392
Acquisitions	4,492
September 30, 2018 - net book value	$	631,884

The carrying amount and accumulated amortization of intangible assets consists of the following as of September 30, 2019 and December 31, 2018 (in thousands):



	September 30, 2019			December 31, 2018
Gross intangible assets:
Referral sources	$	438,445		$	257,792
Trademarks/names	44,702			32,000
Other amortizable intangible assets	379			4,151
Total gross intangible assets	483,526			293,943

Accumulated amortization:
Referral sources	(77,749		)	(63,353		)
Trademarks/names	(10,657		)	(8,000		)
Other amortizable intangible assets	(42		)	(2,877		)
Total accumulated amortization	(88,448		)	(74,230		)
Total intangible assets, net	$	395,078		$	219,713

Amortization expense for intangible assets was $7.5 million and $17.3 million for the three and nine months ended September 30, 2019, respectively. Amortization expense for intangible assets was $4.9 million and $14.7 million for the three and nine months ended September 30, 2018, respectively.

The weighted average amortization period of intangible assets by class and in total as of September 30, 2019 are as follows: 17.1 years for referral sources, 4.1 years for trademarks/names, 1.5 years for other amortizable intangible assets, and 15.9 years for total intangible assets.