Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Total assets	$ 978,866	$ 1,112,346
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	0	0
Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Total assets	1,355,248	1,621,733
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	3,345	814
Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Total assets	713	393
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	307,466	556,520
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	671,400	555,826
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	5,844	2,784
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	299
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	36,699
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	661,457	807,299
Available-for-sale Securities [Member] | Corporate Securities [Member] | Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	713	393
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	368,016	439,064
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	0
Available-for-sale Securities [Member] | Municipal securities [Member] | Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	2,508	3,965
Available-for-sale Securities [Member] | Municipal securities [Member] | Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0	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	280,117	366,123
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	308	2,498
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	3,345	814
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	2,334,827	2,734,472
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	3,345	814
Estimate of Fair Value Measurement [Member] | Cash and Cash Equivalents [Member] | Cash [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	307,466	556,520
Estimate of Fair Value Measurement [Member] | Cash and Cash Equivalents [Member] | Money Market Funds [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	671,400	555,826
Estimate of Fair Value Measurement [Member] | Cash and Cash Equivalents [Member] | Corporate Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	5,844	2,784
Estimate of Fair Value Measurement [Member] | Cash and Cash Equivalents [Member] | Agency Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	299
Estimate of Fair Value Measurement [Member] | Cash and Cash Equivalents [Member] | Government securities [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	36,699
Estimate of Fair Value Measurement [Member] | Available-for-sale Securities [Member] | Corporate Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	662,170	807,692
Estimate of Fair Value Measurement [Member] | Available-for-sale Securities [Member] | Agency Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	368,016	439,064
Estimate of Fair Value Measurement [Member] | Available-for-sale Securities [Member] | Municipal securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	2,508	3,965
Estimate of Fair Value Measurement [Member] | Available-for-sale Securities [Member] | Government securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	280,117	366,123
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	308	2,498
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	$ 3,345	$ 814