Fair Value Measurements, Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Mar. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	$ 1,312,906	$ 501,545
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	1,984,985	1,410,229
Marketable equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	2,465	1,586
Corporate bonds and debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	490,367	207,772
Government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	810,131	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	110,669	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	1,874,316	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	108,204	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	563,875	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	108,204	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	563,875	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	2,465	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	288,224	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	136,825	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		10,043
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	2,465	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	288,224	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	136,825	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		10,043
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	202,143	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	673,306	64,892
Other Long-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	9,943
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 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
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	202,143	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	673,306	$ 64,892
Other Long-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	$ 9,943