Fair Value Measurements, Measured on Recurring Basis (Details) - USD ($) $ in Thousands	6 Months Ended
	Sep. 30, 2015	Mar. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	$ 2,047,376	$ 1,734,380
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	2,597,417	2,351,123
Marketable equity securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value		13,166
Corporate bonds and debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	1,127,283	926,929
Time deposits [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value		506
Government agency bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	868,194	742,256
Municipal bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	42,074	41,698
Insurance sector auction rate securities [Member]
Available-for-sale Securities, Other Disclosure Items [Abstract]
Value of auction rate securities for which recent auctions were unsuccessful	9,800
Auction Rate Securities Par Value	$ 22,400
Insurance sector auction rate securities [Member] | Minimum [Member]
Available-for-sale Securities, Other Disclosure Items [Abstract]
Discount rate	2.00%
Liquidity risk premium (percentage)	9.10%
Anticipated liquidity horizon	7 years
Insurance sector auction rate securities [Member] | Maximum [Member]
Available-for-sale Securities, Other Disclosure Items [Abstract]
Discount rate	2.50%
Liquidity risk premium (percentage)	29.50%
Anticipated liquidity horizon	10 years
Auction rate securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	$ 9,825	9,825
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	217,281	292,999
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	2,364,121	2,042,109
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	16,015	16,015
Cash Equivalents [Member] | Money market mutual funds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash and cash equivalents	217,281	279,833
Cash Equivalents [Member] | Deposit accounts [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash and cash equivalents	321,797	327,982
Cash Equivalents [Member] | Fair Value, Inputs, Level 1 [Member] | Money market mutual funds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash and cash equivalents	217,281	279,833
Cash Equivalents [Member] | Fair Value, Inputs, Level 1 [Member] | Deposit accounts [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash and cash equivalents	0	0
Cash Equivalents [Member] | Fair Value, Inputs, Level 2 [Member] | Money market mutual funds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash and cash equivalents	0	0
Cash Equivalents [Member] | Fair Value, Inputs, Level 2 [Member] | Deposit accounts [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash and cash equivalents	321,797	327,982
Cash Equivalents [Member] | Fair Value, Inputs, Level 3 [Member] | Money market mutual funds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash and cash equivalents	0	0
Cash Equivalents [Member] | Fair Value, Inputs, Level 3 [Member] | Deposit accounts [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash and cash equivalents	0	0
Short-term Investments [Member] | Marketable equity securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value		13,166
Short-term Investments [Member] | Corporate bonds and debt [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	737,194	756,664
Short-term Investments [Member] | Time deposits [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value		506
Short-term Investments [Member] | Government agency bonds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	717,419	549,737
Short-term Investments [Member] | Municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	7,853	30,981
Short-term Investments [Member] | Fair Value, Inputs, Level 1 [Member] | Marketable equity securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value		13,166
Short-term Investments [Member] | Fair Value, Inputs, Level 1 [Member] | Corporate bonds and debt [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	0	0
Short-term Investments [Member] | Fair Value, Inputs, Level 1 [Member] | Time deposits [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value		0
Short-term Investments [Member] | Fair Value, Inputs, Level 1 [Member] | Government agency bonds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	0	0
Short-term Investments [Member] | Fair Value, Inputs, Level 1 [Member] | Municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	0	0
Short-term Investments [Member] | Fair Value, Inputs, Level 2 [Member] | Marketable equity securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value		0
Short-term Investments [Member] | Fair Value, Inputs, Level 2 [Member] | Corporate bonds and debt [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	737,194	756,664
Short-term Investments [Member] | Fair Value, Inputs, Level 2 [Member] | Time deposits [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value		506
Short-term Investments [Member] | Fair Value, Inputs, Level 2 [Member] | Government agency bonds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	717,419	549,737
Short-term Investments [Member] | Fair Value, Inputs, Level 2 [Member] | Municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	7,853	30,981
Short-term Investments [Member] | Fair Value, Inputs, Level 3 [Member] | Marketable equity securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value		0
Short-term Investments [Member] | Fair Value, Inputs, Level 3 [Member] | Corporate bonds and debt [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	0	0
Short-term Investments [Member] | Fair Value, Inputs, Level 3 [Member] | Time deposits [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value		0
Short-term Investments [Member] | Fair Value, Inputs, Level 3 [Member] | Government agency bonds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	0	0
Short-term Investments [Member] | Fair Value, Inputs, Level 3 [Member] | Municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	0	0
Other Long-term Investments [Member] | Corporate bonds and debt [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	390,089	170,265
Other Long-term Investments [Member] | Government agency bonds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	150,775	192,519
Other Long-term Investments [Member] | Municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	34,221	10,717
Other Long-term Investments [Member] | Auction rate securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	9,825	9,825
Other Long-term Investments [Member] | Fair Value, Inputs, Level 1 [Member] | Corporate bonds and debt [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	0	0
Other Long-term Investments [Member] | Fair Value, Inputs, Level 1 [Member] | Government agency bonds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	0	0
Other Long-term Investments [Member] | Fair Value, Inputs, Level 1 [Member] | Municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	0	0
Other Long-term Investments [Member] | Fair Value, Inputs, Level 1 [Member] | Auction rate securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	0	0
Other Long-term Investments [Member] | Fair Value, Inputs, Level 2 [Member] | Corporate bonds and debt [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	383,899	164,075
Other Long-term Investments [Member] | Fair Value, Inputs, Level 2 [Member] | Government agency bonds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	150,775	192,519
Other Long-term Investments [Member] | Fair Value, Inputs, Level 2 [Member] | Municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	34,221	10,717
Other Long-term Investments [Member] | Fair Value, Inputs, Level 2 [Member] | Auction rate securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	0	0
Other Long-term Investments [Member] | Fair Value, Inputs, Level 3 [Member] | Corporate bonds and debt [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	6,190	6,190
Other Long-term Investments [Member] | Fair Value, Inputs, Level 3 [Member] | Government agency bonds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	0	0
Other Long-term Investments [Member] | Fair Value, Inputs, Level 3 [Member] | Municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	0	0
Other Long-term Investments [Member] | Fair Value, Inputs, Level 3 [Member] | Auction rate securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale, estimated fair value	9,825	9,825
Other Noncurrent Assets [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Asset, Fair Value	10,963	8,928
Other Noncurrent Assets [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Asset, Fair Value	0	0
Other Noncurrent Assets [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Asset, Fair Value	10,963	8,928
Other Noncurrent Assets [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Asset, Fair Value	$ 0	$ 0