This item represents the net total realized and unrealized gain (loss) included in earnings for the period as a result of selling or holding marketable securities categorized as trading, available-for-sale, or held-to-maturity, including the unrealized holding gain (loss) of held-to-maturity securities transferred to the trading security category and the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) for available-for-sale securities transferred to trading securities during the period. Additionally, this item would include any gains (losses) realized during the period from the sale of investments accounted for under the cost method of accounting and losses recognized for other than temporary impairments (OTTI) of the subject investments.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

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-Publisher FASB

-Name Accounting Standards Codification

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-Section 50

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-Publisher FASB

-Name Accounting Standards Codification

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Reference 5: http://www.xbrl.org/2003/role/presentationRef

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Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Jun. 29, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash Equivalents and Investments [Line Items]
Amortized Cost		$ 1,700,666	$ 1,399,712
Unrealized Gain		441	371
Unrealized Loss		(845)	(243)
Fair Value		1,700,262	1,399,840
Debt securities due in one year or less		872,872	1,011,459
Due after one year through three years		763,003	314,184
Total debt securities		1,635,875wat_DebtSecurities	1,325,643wat_DebtSecurities
Loss on investments	(1,575)us-gaap_GainLossOnInvestments		(1,575)us-gaap_GainLossOnInvestments
Realized investment gains losses, description		Realized gains and losses on sales of investments were not material in 2014, 2013 and 2012.	Realized gains and losses on sales of investments were not material in 2014, 2013 and 2012.	Realized gains and losses on sales of investments were not material in 2014, 2013 and 2012.
Cash Equivalents [Member]
Cash Equivalents and Investments [Line Items]
Amortized Cost		67,051	36,966
Fair Value		67,051	36,966
Investments [Member]
Cash Equivalents and Investments [Line Items]
Amortized Cost		1,633,615	1,362,746
Unrealized Gain		441	371
Unrealized Loss		(845)	(243)
Fair Value		1,633,211	1,362,874
US Treasury Securities
Cash Equivalents and Investments [Line Items]
Amortized Cost		626,683	556,438
Unrealized Gain		246	111
Unrealized Loss		(157)	(10)
Fair Value		626,772	556,539
Foreign Government Debt Securities
Cash Equivalents and Investments [Line Items]
Amortized Cost		24,998	139,670
Fair Value		24,998	139,670
Corporate Debt Securities
Cash Equivalents and Investments [Line Items]
Amortized Cost		984,668	629,477
Unrealized Gain		125	190
Unrealized Loss		(688)	(233)
Fair Value		984,105	629,434
Time Deposits
Cash Equivalents and Investments [Line Items]
Amortized Cost		64,240	74,050
Fair Value		64,240	74,050
Equity Securities
Cash Equivalents and Investments [Line Items]
Amortized Cost		77	77
Unrealized Gain		70	70
Fair Value		$ 147	$ 147