Fair Value Measurements (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - Recurring basis - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets, Fair Value Disclosure [Abstract]
Total assets	$ 516,775	$ 593,489
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	0	0
Significant Other Observable Inputs (Level 2)
Assets, Fair Value Disclosure [Abstract]
Total assets	89,570	1,183,262
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	1,390	2,543
Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Total assets	1,000	713
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	0	0
Cash and cash equivalents | Cash | Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	474,756	505,363
Cash and cash equivalents | Cash | Significant Other Observable Inputs (Level 2)
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	0
Cash and cash equivalents | Cash | Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	0
Cash and cash equivalents | Money market funds | Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	42,019	88,126
Cash and cash equivalents | Money market funds | Significant Other Observable Inputs (Level 2)
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	0
Cash and cash equivalents | Money market funds | Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	0
Cash and cash equivalents | Agency securities | Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	0
Cash and cash equivalents | Agency securities | Significant Other Observable Inputs (Level 2)
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	19,993	3,296
Cash and cash equivalents | Agency securities | Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	0
Cash and cash equivalents | Corporate securities | Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	0
Cash and cash equivalents | Corporate securities | Significant Other Observable Inputs (Level 2)
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	8,993	9,371
Cash and cash equivalents | Corporate securities | Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	0
Cash and cash equivalents | Government securities | Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents		0
Cash and cash equivalents | Government securities | Significant Other Observable Inputs (Level 2)
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	12,610
Cash and cash equivalents | Government securities | Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0
Available-for-sale securities | Agency securities | Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0	0
Available-for-sale securities | Agency securities | Significant Other Observable Inputs (Level 2)
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	1,682	312,948
Available-for-sale securities | Agency securities | Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0	0
Available-for-sale securities | Corporate securities | Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0	0
Available-for-sale securities | Corporate securities | Significant Other Observable Inputs (Level 2)
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	47,884	607,945
Available-for-sale securities | Corporate securities | Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	1,000	713
Available-for-sale securities | Municipal securities | Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0	0
Available-for-sale securities | Municipal securities | Significant Other Observable Inputs (Level 2)
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0	2,504
Available-for-sale securities | Municipal securities | Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0	0
Available-for-sale securities | Government securities | Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0	0
Available-for-sale securities | Government securities | Significant Other Observable Inputs (Level 2)
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	9,129	233,824
Available-for-sale securities | Government securities | Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0	0
Accrued expenses and other current liabilities | Quoted Prices In Active Markets for Identical Assets (Level 1)
Liabilities, Fair Value Disclosure [Abstract]
Foreign currency derivatives	0	0
Accrued expenses and other current liabilities | Significant Other Observable Inputs (Level 2)
Liabilities, Fair Value Disclosure [Abstract]
Foreign currency derivatives	1,390	2,543
Accrued expenses and other current liabilities | Significant Unobservable Inputs (Level 3)
Liabilities, Fair Value Disclosure [Abstract]
Foreign currency derivatives	0	0
Estimate of Fair Value Measurement
Assets, Fair Value Disclosure [Abstract]
Total assets	607,345	1,777,464
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	1,390	2,543
Estimate of Fair Value Measurement | Cash and cash equivalents | Cash
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	474,756	505,363
Estimate of Fair Value Measurement | Cash and cash equivalents | Money market funds
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	42,019	88,126
Estimate of Fair Value Measurement | Cash and cash equivalents | Agency securities
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	19,993	3,296
Estimate of Fair Value Measurement | Cash and cash equivalents | Corporate securities
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	8,993	9,371
Estimate of Fair Value Measurement | Cash and cash equivalents | Government securities
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	12,610
Estimate of Fair Value Measurement | Available-for-sale securities | Agency securities
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	1,682	312,948
Estimate of Fair Value Measurement | Available-for-sale securities | Corporate securities
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	48,884	608,658
Estimate of Fair Value Measurement | Available-for-sale securities | Municipal securities
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0	2,504
Estimate of Fair Value Measurement | Available-for-sale securities | Government securities
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	9,129	233,824
Estimate of Fair Value Measurement | Accrued expenses and other current liabilities
Liabilities, Fair Value Disclosure [Abstract]
Foreign currency derivatives	1,390	2,543
Foreign currency | Prepaid expenses and other current assets | Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets, Fair Value Disclosure [Abstract]
Derivative assets fair value	0	0
Foreign currency | Prepaid expenses and other current assets | Significant Other Observable Inputs (Level 2)
Assets, Fair Value Disclosure [Abstract]
Derivative assets fair value	1,889	764
Foreign currency | Prepaid expenses and other current assets | Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Derivative assets fair value	0	0
Foreign currency | Estimate of Fair Value Measurement | Prepaid expenses and other current assets
Assets, Fair Value Disclosure [Abstract]
Derivative assets fair value	$ 1,889	$ 764