Fair Value Measurements (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - Recurring basis - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets, Fair Value Disclosure [Abstract]
Total assets	$ 1,112,346	$ 753,402
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	0	0
Significant Other Observable Inputs (Level 2)
Assets, Fair Value Disclosure [Abstract]
Total assets	1,621,733	1,791,266
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	814	4,435
Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Total assets	393	998
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	556,520	528,637
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	555,826	224,765
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 | 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	2,784	82,693
Cash and cash equivalents | Corporate securities | Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	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	439,064	411,493
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	807,299	839,968
Available-for-sale securities | Corporate securities | Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	393	998
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	3,965	9,988
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	366,123	444,618
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	814	4,435
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	2,734,472	2,545,666
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	814	4,435
Estimate of Fair Value Measurement | Cash and cash equivalents | Cash
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	556,520	528,637
Estimate of Fair Value Measurement | Cash and cash equivalents | Money market funds
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	555,826	224,765
Estimate of Fair Value Measurement | Cash and cash equivalents | Corporate securities
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	2,784	82,693
Estimate of Fair Value Measurement | Available-for-sale securities | Agency securities
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	439,064	411,493
Estimate of Fair Value Measurement | Available-for-sale securities | Corporate securities
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	807,692	840,966
Estimate of Fair Value Measurement | Available-for-sale securities | Municipal securities
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	3,965	9,988
Estimate of Fair Value Measurement | Available-for-sale securities | Government securities
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	366,123	444,618
Estimate of Fair Value Measurement | Accrued expenses and other current liabilities
Liabilities, Fair Value Disclosure [Abstract]
Foreign currency derivatives	814	4,435
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	2,498	2,506
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	$ 2,498	$ 2,506