Fair Value Measures (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017 | Jun. 30, 2017 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | $ 618,669 | $ 472,969 | $ 484,537 |
Total liabilities measured at fair value | 3,222 | 14,385 | 5,732 |
Level 1 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 217,056 | 346,425 | 383,830 |
Total liabilities measured at fair value | 0 | 0 | 0 |
Level 2 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 401,613 | 126,544 | 100,707 |
Total liabilities measured at fair value | 3,222 | 14,385 | 5,732 |
Level 3 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 0 | 0 | 0 |
Total liabilities measured at fair value | 0 | 0 | 0 |
Accrued Liabilities [Member] | Derivative Financial Instruments, Liabilities [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total liabilities measured at fair value | 2,895 | 10,565 | 4,067 |
Accrued Liabilities [Member] | Derivative Financial Instruments, Liabilities [Member] | Level 1 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total liabilities measured at fair value | 0 | 0 | 0 |
Accrued Liabilities [Member] | Derivative Financial Instruments, Liabilities [Member] | Level 2 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total liabilities measured at fair value | 2,895 | 10,565 | 4,067 |
Accrued Liabilities [Member] | Derivative Financial Instruments, Liabilities [Member] | Level 3 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total liabilities measured at fair value | 0 | 0 | 0 |
Other Long Term Liabilities [Member] | Derivative Financial Instruments, Liabilities [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total liabilities measured at fair value | 327 | 3,820 | 1,665 |
Other Long Term Liabilities [Member] | Derivative Financial Instruments, Liabilities [Member] | Level 1 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total liabilities measured at fair value | 0 | 0 | 0 |
Other Long Term Liabilities [Member] | Derivative Financial Instruments, Liabilities [Member] | Level 2 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total liabilities measured at fair value | 327 | 3,820 | 1,665 |
Other Long Term Liabilities [Member] | Derivative Financial Instruments, Liabilities [Member] | Level 3 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total liabilities measured at fair value | 0 | 0 | 0 |
Cash Equivalents [Member] | Money Market Funds [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 144,729 | 282,860 | 307,724 |
Cash Equivalents [Member] | Money Market Funds [Member] | Level 1 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 144,729 | 282,860 | 307,724 |
Cash Equivalents [Member] | Money Market Funds [Member] | Level 2 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 0 | 0 | 0 |
Cash Equivalents [Member] | Money Market Funds [Member] | Level 3 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 0 | 0 | 0 |
Cash Equivalents [Member] | Time Deposits [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 61,563 | 52,808 | 66,217 |
Cash Equivalents [Member] | Time Deposits [Member] | Level 1 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 61,563 | 52,808 | 66,217 |
Cash Equivalents [Member] | Time Deposits [Member] | Level 2 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 0 | | |
Cash Equivalents [Member] | Time Deposits [Member] | Level 3 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 0 | 0 | 0 |
Cash Equivalents [Member] | Repurchase Agreements [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 121,352 | 4,995 | |
Cash Equivalents [Member] | Repurchase Agreements [Member] | Level 1 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 0 | 0 | |
Cash Equivalents [Member] | Repurchase Agreements [Member] | Level 2 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 121,352 | 4,995 | |
Cash Equivalents [Member] | Repurchase Agreements [Member] | Level 3 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 0 | 0 | |
Cash Equivalents [Member] | US Government Backed Municipal Bonds [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | | 25,338 | 97,738 |
Cash Equivalents [Member] | US Government Backed Municipal Bonds [Member] | Level 1 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | | 0 | 0 |
Cash Equivalents [Member] | US Government Backed Municipal Bonds [Member] | Level 2 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | | 25,338 | 97,738 |
Cash Equivalents [Member] | US Government Backed Municipal Bonds [Member] | Level 3 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | | 0 | 0 |
Available-For-Sale Short-Term Investments [Member] | Repurchase Agreements [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 225,247 | 19,963 | |
Available-For-Sale Short-Term Investments [Member] | Repurchase Agreements [Member] | Level 1 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 0 | 0 | |
Available-For-Sale Short-Term Investments [Member] | Repurchase Agreements [Member] | Level 2 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 225,247 | 19,963 | |
Available-For-Sale Short-Term Investments [Member] | Repurchase Agreements [Member] | Level 3 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 0 | 0 | |
Available-For-Sale Short-Term Investments [Member] | US Government Backed Municipal Bonds [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 37,335 | 73,582 | |
Available-For-Sale Short-Term Investments [Member] | US Government Backed Municipal Bonds [Member] | Level 1 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 0 | 0 | |
Available-For-Sale Short-Term Investments [Member] | US Government Backed Municipal Bonds [Member] | Level 2 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 37,335 | 73,582 | |
Available-For-Sale Short-Term Investments [Member] | US Government Backed Municipal Bonds [Member] | Level 3 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 0 | 0 | |
Short-term Investments [Member] | Mutual Fund Shares [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 1,432 | 1,438 | 1,591 |
Short-term Investments [Member] | Mutual Fund Shares [Member] | Level 1 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 1,432 | 1,438 | 1,591 |
Short-term Investments [Member] | Mutual Fund Shares [Member] | Level 2 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 0 | 0 | 0 |
Short-term Investments [Member] | Mutual Fund Shares [Member] | Level 3 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 0 | 0 | 0 |
Other Current Assets [Member] | Derivative Financial Instruments, Assets [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 7,163 | 2,331 | 2,153 |
Other Current Assets [Member] | Derivative Financial Instruments, Assets [Member] | Level 1 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 0 | 0 | 0 |
Other Current Assets [Member] | Derivative Financial Instruments, Assets [Member] | Level 2 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 7,163 | 2,331 | 2,153 |
Other Current Assets [Member] | Derivative Financial Instruments, Assets [Member] | Level 3 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 0 | 0 | 0 |
Non Current Assets [Member] | Mutual Fund Shares [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 9,332 | 9,319 | 8,298 |
Non Current Assets [Member] | Mutual Fund Shares [Member] | Level 1 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 9,332 | 9,319 | 8,298 |
Non Current Assets [Member] | Mutual Fund Shares [Member] | Level 2 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 0 | 0 | 0 |
Non Current Assets [Member] | Mutual Fund Shares [Member] | Level 3 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 0 | 0 | 0 |
Non Current Assets [Member] | Derivative Financial Instruments, Assets [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 10,516 | 335 | 816 |
Non Current Assets [Member] | Derivative Financial Instruments, Assets [Member] | Level 1 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 0 | 0 | |
Non Current Assets [Member] | Derivative Financial Instruments, Assets [Member] | Level 2 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | 10,516 | 335 | $ 816 |
Non Current Assets [Member] | Derivative Financial Instruments, Assets [Member] | Level 3 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Total assets measured at fair value | $ 0 | $ 0 | |