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Measured on Recurring Basis (Details) (USD $)
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6 Months Ended | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
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Sep. 30, 2014
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Mar. 31, 2014
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Sep. 30, 2013
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Mar. 31, 2013
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Sep. 30, 2014
Insurance Sector Auction Rate Securities [Member]
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Sep. 30, 2014
Insurance Sector Auction Rate Securities [Member]
Minimum [Member]
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Sep. 30, 2014
Insurance Sector Auction Rate Securities [Member]
Maximum [Member]
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Sep. 30, 2014
Fair Value, Inputs, Level 1 [Member]
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Mar. 31, 2014
Fair Value, Inputs, Level 1 [Member]
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Sep. 30, 2014
Fair Value, Inputs, Level 2 [Member]
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Mar. 31, 2014
Fair Value, Inputs, Level 2 [Member]
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Sep. 30, 2014
Fair Value, Inputs, Level 3 [Member]
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Mar. 31, 2014
Fair Value, Inputs, Level 3 [Member]
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| Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items] | |||||||||||||
| Money Market Fund Deposits Fair Value Disclosure | $ 95,464,000 | $ 192,159,000 | $ 95,464,000 | $ 192,159,000 | $ 0 | $ 0 | $ 0 | $ 0 | |||||
| Corporate Bonds and Debt Fair Value Disclosure | 905,593,000 | 943,966,000 | 0 | 0 | 899,403,000 | 937,776,000 | 6,190,000 | 6,190,000 | |||||
| Time Deposits Fair Value Disclosure | 20,160,000 | 0 | 20,160,000 | 0 | |||||||||
| Government Agency Bonds Fair Value Disclosure | 666,208,000 | 681,394,000 | 0 | 0 | 666,208,000 | 681,394,000 | 0 | 0 | |||||
| Cash Deposit Accounts Fair Value Disclosure | 385,975,000 | 274,444,000 | 0 | 0 | 385,975,000 | 274,444,000 | 0 | 0 | |||||
| Municipal Bonds Fair Value Disclosure | 41,161,000 | 41,709,000 | 0 | 0 | 41,161,000 | 41,709,000 | 0 | 0 | |||||
| Auction Rate Securities Fair Value Disclosure | 9,825,000 | 9,825,000 | 0 | 0 | 0 | 0 | 9,825,000 | 9,825,000 | |||||
| Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract] | |||||||||||||
| Cash and cash equivalents | 481,439,000 | 466,603,000 | 344,202,000 | 528,334,000 | 95,464,000 | 192,159,000 | 385,975,000 | 274,444,000 | 0 | 0 | |||
| Short-term investments | 801,670,000 | 878,182,000 | 0 | 0 | 801,670,000 | 878,182,000 | 0 | 0 | |||||
| Long-term investments | 841,277,000 | 798,712,000 | 0 | 0 | 825,262,000 | 782,697,000 | 16,015,000 | 16,015,000 | |||||
| Total assets measured at fair value | 2,124,386,000 | 2,143,497,000 | 95,464,000 | 192,159,000 | 2,012,907,000 | 1,935,323,000 | 16,015,000 | 16,015,000 | |||||
| Available-for-sale Securities, Other Disclosure Items [Abstract] | |||||||||||||
| Value of auction rate securities for which recent auctions were unsuccessful | 9,800,000 | ||||||||||||
| Discount rate | 2.00% | 2.50% | |||||||||||
| Auction Rate Securities Par Value | $ 22,400,000 | ||||||||||||
| Liquidity risk premium (percentage) | 9.10% | 29.50% | |||||||||||
| Anticipated liquidity horizon | 7 years | 10 years | |||||||||||