Fair Value Measurements (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Recurring basis | Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets, Fair Value Disclosure [Abstract]
Total assets	$ 364,930	$ 259,882
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	0	0
Recurring basis | Significant Other Observable Inputs (Level 2)
Assets, Fair Value Disclosure [Abstract]
Total assets	1,397,905	1,597,770
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	3,678	9,692
Recurring basis | Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Total assets	1,562	6,073
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	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
Cash and cash equivalents | Corporate securities | Significant Other Observable Inputs (Level 2)
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	3,588
Cash and cash equivalents | Corporate securities | Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0
Cash and cash equivalents | Recurring basis | Cash | Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	261,962	230,370
Cash and cash equivalents | Recurring basis | Cash | Significant Other Observable Inputs (Level 2)
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	0
Cash and cash equivalents | Recurring basis | Cash | Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	0
Cash and cash equivalents | Recurring basis | Money market funds | Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	102,968	29,512
Cash and cash equivalents | Recurring basis | Money market funds | Significant Other Observable Inputs (Level 2)
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	0
Cash and cash equivalents | Recurring basis | Money market funds | Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	0
Cash and cash equivalents | Recurring basis | Corporate 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 | Recurring basis | Corporate securities | Significant Other Observable Inputs (Level 2)
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents		267
Cash and cash equivalents | Recurring basis | Corporate securities | Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents		0
Available-for-sale securities | Recurring basis | 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 | Recurring basis | Corporate securities | Significant Other Observable Inputs (Level 2)
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	695,809	788,042
Available-for-sale securities | Recurring basis | Corporate securities | Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	1,562	6,073
Available-for-sale securities | Recurring basis | 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 | Recurring basis | Agency securities | Significant Other Observable Inputs (Level 2)
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	530,522	638,313
Available-for-sale securities | Recurring basis | Agency securities | Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0	0
Available-for-sale securities | Recurring basis | 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 | Recurring basis | Municipal securities | Significant Other Observable Inputs (Level 2)
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	14,878	48,730
Available-for-sale securities | Recurring basis | Municipal securities | Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0	0
Available-for-sale securities | Recurring basis | 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 | Recurring basis | Government securities | Significant Other Observable Inputs (Level 2)
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	152,045	121,212
Available-for-sale securities | Recurring basis | Government securities | Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0	0
Accrued expenses and other current liabilities | Recurring basis | 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 | Recurring basis | Significant Other Observable Inputs (Level 2)
Liabilities, Fair Value Disclosure [Abstract]
Foreign currency derivatives	3,678	9,692
Accrued expenses and other current liabilities | Recurring basis | Significant Unobservable Inputs (Level 3)
Liabilities, Fair Value Disclosure [Abstract]
Foreign currency derivatives	0	0
Estimate of Fair Value Measurement | Recurring basis
Assets, Fair Value Disclosure [Abstract]
Total assets	1,764,397	1,863,725
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	3,678	9,692
Estimate of Fair Value Measurement | Cash and cash equivalents | Corporate securities
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	3,588
Estimate of Fair Value Measurement | Cash and cash equivalents | Recurring basis | Cash
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	261,962	230,370
Estimate of Fair Value Measurement | Cash and cash equivalents | Recurring basis | Money market funds
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	102,968	29,512
Estimate of Fair Value Measurement | Cash and cash equivalents | Recurring basis | Corporate securities
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents		267
Estimate of Fair Value Measurement | Available-for-sale securities | Recurring basis | Corporate securities
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	697,371	794,115
Estimate of Fair Value Measurement | Available-for-sale securities | Recurring basis | Agency securities
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	530,522	638,313
Estimate of Fair Value Measurement | Available-for-sale securities | Recurring basis | Municipal securities
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	14,878	48,730
Estimate of Fair Value Measurement | Available-for-sale securities | Recurring basis | Government securities
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	152,045	121,212
Estimate of Fair Value Measurement | Accrued expenses and other current liabilities | Recurring basis
Liabilities, Fair Value Disclosure [Abstract]
Foreign currency derivatives	3,678	9,692
Foreign currency | Prepaid expenses and other current assets | Recurring basis | 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 | Recurring basis | Significant Other Observable Inputs (Level 2)
Assets, Fair Value Disclosure [Abstract]
Derivative assets fair value	1,063	1,206
Foreign currency | Prepaid expenses and other current assets | Recurring basis | 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 | Recurring basis
Assets, Fair Value Disclosure [Abstract]
Derivative assets fair value	$ 1,063	$ 1,206