Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Total assets	$ 936,015,000		$ 936,015,000		$ 753,402,000
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	0		0		0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Total assets	1,626,836,000		1,626,836,000		1,791,266,000
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	622,000		622,000		4,435,000
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Total assets	394,000		394,000		998,000
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	0		0		0
Cash and Cash Equivalents [Member] | Fair Value, Measurements, Recurring [Member] | Cash [Member] | Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	869,770,000		869,770,000		528,637,000
Cash and Cash Equivalents [Member] | Fair Value, Measurements, Recurring [Member] | Cash [Member] | Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0		0		0
Cash and Cash Equivalents [Member] | Fair Value, Measurements, Recurring [Member] | Cash [Member] | Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0		0		0
Cash and Cash Equivalents [Member] | Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	66,245,000		66,245,000		224,765,000
Cash and Cash Equivalents [Member] | Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0		0		0
Cash and Cash Equivalents [Member] | Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0		0		0
Cash and Cash Equivalents [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Securities [Member] | Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0		0		0
Cash and Cash Equivalents [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Securities [Member] | Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	4,854,000		4,854,000		82,693,000
Cash and Cash Equivalents [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Securities [Member] | Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0		0		0
Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Securities [Member] | Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0		0		0
Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Securities [Member] | Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	802,086,000		802,086,000		839,968,000
Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Securities [Member] | Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	394,000		394,000		998,000
Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Agency Securities [Member] | Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0		0		0
Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Agency Securities [Member] | Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	482,658,000		482,658,000		411,493,000
Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Agency Securities [Member] | Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0		0		0
Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Municipal securities [Member] | Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0		0		0
Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Municipal securities [Member] | Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	3,977,000		3,977,000		9,988,000
Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Municipal securities [Member] | Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0		0		0
Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Government securities [Member] | Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0		0		0
Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Government securities [Member] | Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	328,730,000		328,730,000		444,618,000
Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Government securities [Member] | Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0		0		0
Foreign Exchange Contract [Member] | Prepaid Expenses and Other Current Assets [Member] | Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Prepaid expenses and other current assets	0		0		0
Foreign Exchange Contract [Member] | Prepaid Expenses and Other Current Assets [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Prepaid expenses and other current assets	4,531,000		4,531,000		2,506,000
Foreign Exchange Contract [Member] | Prepaid Expenses and Other Current Assets [Member] | Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Prepaid expenses and other current assets	0		0		0
Foreign Exchange Contract [Member] | Accrued Expenses and Other Current Liabilities [Member] | Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Liabilities, Fair Value Disclosure [Abstract]
Foreign currency derivatives	0		0		0
Foreign Exchange Contract [Member] | Accrued Expenses and Other Current Liabilities [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Liabilities, Fair Value Disclosure [Abstract]
Foreign currency derivatives	622,000		622,000		4,435,000
Foreign Exchange Contract [Member] | Accrued Expenses and Other Current Liabilities [Member] | Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Liabilities, Fair Value Disclosure [Abstract]
Foreign currency derivatives	0		0		0
Estimate of Fair Value Measurement [Member] | Fair Value, Measurements, Recurring [Member]
Assets, Fair Value Disclosure [Abstract]
Total assets	2,563,245,000		2,563,245,000		2,545,666,000
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	622,000		622,000		4,435,000
Estimate of Fair Value Measurement [Member] | Cash and Cash Equivalents [Member] | Fair Value, Measurements, Recurring [Member] | Cash [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	869,770,000		869,770,000		528,637,000
Estimate of Fair Value Measurement [Member] | Cash and Cash Equivalents [Member] | Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	66,245,000		66,245,000		224,765,000
Estimate of Fair Value Measurement [Member] | Cash and Cash Equivalents [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	4,854,000		4,854,000		82,693,000
Estimate of Fair Value Measurement [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	802,480,000		802,480,000		840,966,000
Estimate of Fair Value Measurement [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Agency Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	482,658,000		482,658,000		411,493,000
Estimate of Fair Value Measurement [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Municipal securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	3,977,000		3,977,000		9,988,000
Estimate of Fair Value Measurement [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Government securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	328,730,000		328,730,000		444,618,000
Estimate of Fair Value Measurement [Member] | Foreign Exchange Contract [Member] | Prepaid Expenses and Other Current Assets [Member] | Fair Value, Measurements, Recurring [Member]
Assets, Fair Value Disclosure [Abstract]
Prepaid expenses and other current assets	4,531,000		4,531,000		2,506,000
Estimate of Fair Value Measurement [Member] | Foreign Exchange Contract [Member] | Accrued Expenses and Other Current Liabilities [Member] | Fair Value, Measurements, Recurring [Member]
Liabilities, Fair Value Disclosure [Abstract]
Foreign currency derivatives	622,000		622,000		$ 4,435,000
Other Income (Expense) [Member] | Fair Value, Measurements, Nonrecurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cost-method Investments, Other than Temporary Impairment	$ 0	$ 900,000	$ 1,400,000	$ 1,100,000