v3.8.0.1
Fair Value Measurements, Measured on Recurring Basis (Details) - USD ($)
$ in Thousands
Sep. 30, 2017
Mar. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available-for-sale, estimated fair value $ 1,017,446 $ 501,545
Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Total assets measured at fair value 1,843,853 1,410,229
Marketable equity securities    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available-for-sale, estimated fair value 2,943 1,586
Corporate bonds and debt    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available-for-sale, estimated fair value 393,535 207,772
Government agency bonds    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available-for-sale, estimated fair value 610,952 226,865
Municipal bonds - tax-exempt    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available-for-sale, estimated fair value   55,279
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Total assets measured at fair value 182,637 345,401
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Total assets measured at fair value 1,661,216 1,064,828
Cash and Cash Equivalents | Money market mutual funds | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Cash and cash equivalents 179,694 343,815
Cash and Cash Equivalents | Deposit accounts | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Cash and cash equivalents 646,713 564,869
Cash and Cash Equivalents | Fair Value, Inputs, Level 1 | Money market mutual funds | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Cash and cash equivalents 179,694 343,815
Cash and Cash Equivalents | Fair Value, Inputs, Level 1 | Deposit accounts | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Cash and cash equivalents 0 0
Cash and Cash Equivalents | Fair Value, Inputs, Level 2 | Money market mutual funds | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Cash and cash equivalents 0 0
Cash and Cash Equivalents | Fair Value, Inputs, Level 2 | Deposit accounts | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Cash and cash equivalents 646,713 564,869
Short-term Investments | Marketable equity securities | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available-for-sale, estimated fair value 2,943 1,586
Short-term Investments | Corporate bonds and debt | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available-for-sale, estimated fair value 293,708 165,207
Short-term Investments | Government agency bonds | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available-for-sale, estimated fair value 181,530 161,973
Short-term Investments | Municipal bonds - tax-exempt | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available-for-sale, estimated fair value   55,279
Short-term Investments | Municipal bonds - taxable | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available-for-sale, estimated fair value 10,016 10,043
Short-term Investments | Fair Value, Inputs, Level 1 | Marketable equity securities | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available-for-sale, estimated fair value 2,943 1,586
Short-term Investments | Fair Value, Inputs, Level 1 | Corporate bonds and debt | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available-for-sale, estimated fair value 0 0
Short-term Investments | Fair Value, Inputs, Level 1 | Government agency bonds | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available-for-sale, estimated fair value 0 0
Short-term Investments | Fair Value, Inputs, Level 1 | Municipal bonds - tax-exempt | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available-for-sale, estimated fair value   0
Short-term Investments | Fair Value, Inputs, Level 1 | Municipal bonds - taxable | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available-for-sale, estimated fair value   0
Short-term Investments | Fair Value, Inputs, Level 2 | Marketable equity securities | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available-for-sale, estimated fair value 0 0
Short-term Investments | Fair Value, Inputs, Level 2 | Corporate bonds and debt | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available-for-sale, estimated fair value 293,708 165,207
Short-term Investments | Fair Value, Inputs, Level 2 | Government agency bonds | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available-for-sale, estimated fair value 181,530 161,973
Short-term Investments | Fair Value, Inputs, Level 2 | Municipal bonds - tax-exempt | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available-for-sale, estimated fair value   55,279
Short-term Investments | Fair Value, Inputs, Level 2 | Municipal bonds - taxable | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available-for-sale, estimated fair value 10,016 10,043
Other Long-term Investments | Corporate bonds and debt | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available-for-sale, estimated fair value 99,827 42,565
Other Long-term Investments | Government agency bonds | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available-for-sale, estimated fair value 429,422 64,892
Other Long-term Investments | Fair Value, Inputs, Level 1 | Corporate bonds and debt | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available-for-sale, estimated fair value 0 0
Other Long-term Investments | Fair Value, Inputs, Level 1 | Government agency bonds | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available-for-sale, estimated fair value 0 0
Other Long-term Investments | Fair Value, Inputs, Level 2 | Corporate bonds and debt | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available-for-sale, estimated fair value 99,827 42,565
Other Long-term Investments | Fair Value, Inputs, Level 2 | Government agency bonds | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available-for-sale, estimated fair value $ 429,422 $ 64,892