Fair Value Measurements (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Recurring basis | Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets, Fair Value Disclosure [Abstract]
Total assets	$ 259,882	$ 280,351
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	0	0
Recurring basis | Significant Other Observable Inputs (Level 2)
Assets, Fair Value Disclosure [Abstract]
Total assets	1,597,770	1,304,727
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	9,692	1,743
Recurring basis | Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Total assets	6,073	10,291
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	267
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	230,370	227,528
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	29,512	52,823
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		389
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	788,042	633,801
Available-for-sale securities | Recurring basis | Corporate securities | Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	6,073	10,291
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	638,313	454,750
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	48,730	53,756
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	121,212	157,079
Available-for-sale securities | Recurring basis | Government securities | Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0	0
Prepaid expenses and other current assets | Recurring basis | Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets, Fair Value Disclosure [Abstract]
Foreign currency derivatives	0	0
Prepaid expenses and other current assets | Recurring basis | Significant Other Observable Inputs (Level 2)
Assets, Fair Value Disclosure [Abstract]
Foreign currency derivatives	1,206	4,952
Prepaid expenses and other current assets | Recurring basis | Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Foreign currency derivatives	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	9,692	1,743
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	1,863,725	1,595,368
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	9,692	1,743
Estimate of Fair Value Measurement | Cash and cash equivalents | Corporate securities
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	267
Estimate of Fair Value Measurement | Cash and cash equivalents | Recurring basis | Cash
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	230,370	227,528
Estimate of Fair Value Measurement | Cash and cash equivalents | Recurring basis | Money market funds
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	29,512	52,823
Estimate of Fair Value Measurement | Cash and cash equivalents | Recurring basis | Corporate securities
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents		389
Estimate of Fair Value Measurement | Available-for-sale securities | Recurring basis | Corporate securities
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	794,115	644,091
Estimate of Fair Value Measurement | Available-for-sale securities | Recurring basis | Agency securities
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	638,313	454,750
Estimate of Fair Value Measurement | Available-for-sale securities | Recurring basis | Municipal securities
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	48,730	53,756
Estimate of Fair Value Measurement | Available-for-sale securities | Recurring basis | Government securities
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	121,212	157,079
Estimate of Fair Value Measurement | Prepaid expenses and other current assets | Recurring basis
Assets, Fair Value Disclosure [Abstract]
Foreign currency derivatives	1,206	4,952
Estimate of Fair Value Measurement | Accrued expenses and other current liabilities | Recurring basis
Liabilities, Fair Value Disclosure [Abstract]
Foreign currency derivatives	$ 9,692	$ 1,743