Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Total assets	$ 1,608,339	$ 593,489
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	0	0
Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Total assets	265,063	1,183,262
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	1,188	2,543
Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Total assets	1,000	713
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	0	0
Cash and Cash Equivalents [Member] | Cash [Member] | Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	691,539	505,363
Cash and Cash Equivalents [Member] | Cash [Member] | Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	0
Cash and Cash Equivalents [Member] | Cash [Member] | Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	0
Cash and Cash Equivalents [Member] | Money Market Funds [Member] | Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	916,800	88,126
Cash and Cash Equivalents [Member] | Money Market Funds [Member] | Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	0
Cash and Cash Equivalents [Member] | Money Market Funds [Member] | Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	0
Cash and Cash Equivalents [Member] | Corporate Securities [Member] | Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	0
Cash and Cash Equivalents [Member] | Corporate Securities [Member] | Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	4,340	9,371
Cash and Cash Equivalents [Member] | Corporate Securities [Member] | Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	0
Cash and Cash Equivalents [Member] | Agency Securities [Member] | Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents		0
Cash and Cash Equivalents [Member] | Agency Securities [Member] | Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents		3,296
Cash and Cash Equivalents [Member] | Agency Securities [Member] | Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents		0
Cash and Cash Equivalents [Member] | Government securities [Member] | Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents		0
Cash and Cash Equivalents [Member] | Government securities [Member] | Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents		12,610
Cash and Cash Equivalents [Member] | Government securities [Member] | Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents		0
Available-for-sale Securities [Member] | Corporate Securities [Member] | Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0	0
Available-for-sale Securities [Member] | Corporate Securities [Member] | Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	117,298	607,945
Available-for-sale Securities [Member] | Corporate Securities [Member] | Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	1,000	713
Available-for-sale Securities [Member] | Agency Securities [Member] | Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0	0
Available-for-sale Securities [Member] | Agency Securities [Member] | Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	69,947	312,948
Available-for-sale Securities [Member] | Agency Securities [Member] | Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0	0
Available-for-sale Securities [Member] | Municipal securities [Member] | Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities		0
Available-for-sale Securities [Member] | Municipal securities [Member] | Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities		2,504
Available-for-sale Securities [Member] | Municipal securities [Member] | Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities		0
Available-for-sale Securities [Member] | Government securities [Member] | Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0	0
Available-for-sale Securities [Member] | Government securities [Member] | Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	71,696	233,824
Available-for-sale Securities [Member] | Government securities [Member] | Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0	0
Foreign Exchange Contract [Member] | Prepaid Expenses and Other Current Assets [Member] | Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Prepaid expenses and other current assets	0	0
Foreign Exchange Contract [Member] | Prepaid Expenses and Other Current Assets [Member] | Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Prepaid expenses and other current assets	1,782	764
Foreign Exchange Contract [Member] | Prepaid Expenses and Other Current Assets [Member] | Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Prepaid expenses and other current assets	0	0
Foreign Exchange Contract [Member] | Accrued Expenses and Other Current Liabilities [Member] | Level 1 [Member]
Liabilities, Fair Value Disclosure [Abstract]
Foreign currency derivatives	0	0
Foreign Exchange Contract [Member] | Accrued Expenses and Other Current Liabilities [Member] | Level 2 [Member]
Liabilities, Fair Value Disclosure [Abstract]
Foreign currency derivatives	1,188	2,543
Foreign Exchange Contract [Member] | Accrued Expenses and Other Current Liabilities [Member] | Level 3 [Member]
Liabilities, Fair Value Disclosure [Abstract]
Foreign currency derivatives	0	0
Estimate of Fair Value Measurement [Member]
Assets, Fair Value Disclosure [Abstract]
Total assets	1,874,402	1,777,464
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	1,188	2,543
Estimate of Fair Value Measurement [Member] | Cash and Cash Equivalents [Member] | Cash [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	691,539	505,363
Estimate of Fair Value Measurement [Member] | Cash and Cash Equivalents [Member] | Money Market Funds [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	916,800	88,126
Estimate of Fair Value Measurement [Member] | Cash and Cash Equivalents [Member] | Corporate Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	4,340	9,371
Estimate of Fair Value Measurement [Member] | Cash and Cash Equivalents [Member] | Agency Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents		3,296
Estimate of Fair Value Measurement [Member] | Cash and Cash Equivalents [Member] | Government securities [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents		12,610
Estimate of Fair Value Measurement [Member] | Available-for-sale Securities [Member] | Corporate Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	118,298	608,658
Estimate of Fair Value Measurement [Member] | Available-for-sale Securities [Member] | Agency Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	69,947	312,948
Estimate of Fair Value Measurement [Member] | Available-for-sale Securities [Member] | Municipal securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities		2,504
Estimate of Fair Value Measurement [Member] | Available-for-sale Securities [Member] | Government securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	71,696	233,824
Estimate of Fair Value Measurement [Member] | Foreign Exchange Contract [Member] | Prepaid Expenses and Other Current Assets [Member]
Assets, Fair Value Disclosure [Abstract]
Prepaid expenses and other current assets	1,782	764
Estimate of Fair Value Measurement [Member] | Foreign Exchange Contract [Member] | Accrued Expenses and Other Current Liabilities [Member]
Liabilities, Fair Value Disclosure [Abstract]
Foreign currency derivatives	$ 1,188	$ 2,543