7. Financial instruments and risk concentration

Financial instruments

We hold derivative financial instruments such as forward foreign currency exchange contracts, the fair value of which was not material as of December 31, 2017. Our forward foreign currency exchange contracts outstanding as of December 31, 2017, had a notional value of $365 million to hedge our non-U.S. dollar net balance sheet exposures, including $140 million to sell Japanese yen, $59 million to sell British pound sterling and $49 million to sell euros.

Our investments in cash equivalents, short-term investments and certain long-term investments, as well as our deferred compensation liabilities, are carried at fair value. Our postretirement plan assets are carried at fair value or net asset value per share. The carrying values for other current financial assets and liabilities, such as accounts receivable and accounts payable, approximate fair value due to the short maturity of such instruments. The carrying value of our long-term debt approximates the fair value as measured using broker-dealer quotes, which are Level 2 inputs. See Note 8 for a description of fair value and the definition of Level 2 inputs.

Risk concentration

We are subject to counterparty risks from financial institutions, customers and issuers of debt securities. Financial instruments that could subject us to concentrations of credit risk are primarily cash deposits, cash equivalents, short-term investments and accounts receivable. To manage our credit risk exposure, we place cash investments in investment-grade debt securities and limit the amount of credit exposure to any one issuer. We also limit counterparties on cash deposits and financial derivative contracts to financial institutions with investment-grade ratings.

Concentrations of credit risk with respect to accounts receivable are limited due to our large number of customers and their dispersion across different industries and geographic areas. We maintain allowances for expected returns, disputes, adjustments, incentives and collectability. These allowances are deducted from accounts receivable on our Consolidated Balance Sheets.

Details of these accounts receivable allowances are as follows:

	2017			2016		2015
Balance, January 1	$	17		$	7	$	12
Amounts charged (credited) to operating results		(9	)		10		(5	)
Recoveries and write-offs, net		—			—		—
Balance, December 31	$	8		$	17	$	7

Major customer

No end customer accounted for 10 percent or more of revenue in 2017 or 2016. In 2015, Apple Inc. accounted for approximately 11 percent of revenue, recognized primarily in our Analog segment.