(10) Amortizable Intangible Assets

The carrying amount and accumulated amortization of intangible assets as of December 31 were as follows:

	2016
	Gross Carrying Amount		Accumulated Amortization			Net Carrying Amount
	(In millions)
Customer relationships and platforms	$	745.2		(197.5	)		547.7
Other		126.9		(35.5	)		91.4
Total	$	872.1		(233.0	)		639.1

	2015
	Gross Carrying Amount		Accumulated Amortization			Net Carrying Amount
	(In millions)
Customer relationships and platforms	$	745.2		(137.0	)		608.2
Other		126.7		(26.0	)		100.7
Total	$	871.9		(163.0	)		708.9

Amortization expense was $69.9 million for 2016, $69.9 million for 2015, and $54.8 million for 2014. As of December 31, 2016, the weighted-average amortization period of customer relationships and platforms was 12 years. Other intangible assets primarily consist of patented and unpatented technology, and trade names and trademarks, which have weighted-average amortization periods of 13 and 15 years, respectively.

Estimated amortization expense for the next five years is $69.9 million for 2017, $68.4 million for 2018 and 2019, $68.3 million for 2020, and $68.2 for 2021.