Measured on Recurring Basis (Details) (USD $)						6 Months Ended
	Sep. 30, 2014	Mar. 31, 2014	Sep. 30, 2013	Mar. 31, 2013	Sep. 30, 2014 Insurance Sector Auction Rate Securities [Member]	Sep. 30, 2014 Insurance Sector Auction Rate Securities [Member] Minimum [Member]	Sep. 30, 2014 Insurance Sector Auction Rate Securities [Member] Maximum [Member]	Sep. 30, 2014 Fair Value, Inputs, Level 1 [Member]	Mar. 31, 2014 Fair Value, Inputs, Level 1 [Member]	Sep. 30, 2014 Fair Value, Inputs, Level 2 [Member]	Mar. 31, 2014 Fair Value, Inputs, Level 2 [Member]	Sep. 30, 2014 Fair Value, Inputs, Level 3 [Member]	Mar. 31, 2014 Fair Value, Inputs, Level 3 [Member]
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