Measured on Recurring Basis (Details) (USD $)							6 Months Ended
	Sep. 30, 2013	Mar. 31, 2013	Sep. 30, 2012	Mar. 31, 2012	Sep. 30, 2013 Insurance Sector Auction Rate Securities [Member]	Mar. 31, 2013 Insurance Sector Auction Rate Securities [Member]	Sep. 30, 2013 Insurance Sector Auction Rate Securities [Member] Minimum [Member]	Sep. 30, 2013 Insurance Sector Auction Rate Securities [Member] Maximum [Member]	Mar. 31, 2013 Energy And Telecommunications Sector Insurance Sector Auction Rate Securities [Member]	Mar. 31, 2013 Student Loan Auction Rate Securities [Member]	Sep. 30, 2013 Fair Value, Inputs, Level 1 [Member]	Mar. 31, 2013 Fair Value, Inputs, Level 1 [Member]	Sep. 30, 2013 Fair Value, Inputs, Level 2 [Member]	Mar. 31, 2013 Fair Value, Inputs, Level 2 [Member]	Sep. 30, 2013 Fair Value, Inputs, Level 3 [Member]	Mar. 31, 2013 Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Money Market Fund Deposits Fair Value Disclosure	$ 49,949,000	$ 100,878,000									$ 49,949,000	$ 100,878,000	$ 0	$ 0	$ 0	$ 0
Marketable Equity Securities Fair Value Disclosure		5,509,000										5,509,000		0		0
Corporate Bonds and Debt Fair Value Disclosure	797,733,000	685,122,000									0	0	791,543,000	678,932,000	6,190,000	6,190,000
Government Agency Bonds Fair Value Disclosure	776,032,000	558,153,000									0	0	776,032,000	558,153,000	0	0
Cash Deposit Accounts Fair Value Disclosure	294,253,000	427,456,000									0	0	294,253,000	427,456,000	0	0
Municipal Bonds Fair Value Disclosure	53,771,000	25,138,000									0	0	53,771,000	25,138,000	0	0
Auction Rate Securities Fair Value Disclosure	9,829,000	33,791,000									0	0	0	0	9,829,000	33,791,000
Contingent consideration	1,022,000	19,100,000									0	0	0	0	1,022,000	19,100,000
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	344,202,000	528,334,000	436,735,000	635,755,000							49,949,000	100,878,000	294,253,000	427,456,000	0	0
Short-term investments	807,709,000	1,050,263,000									0	0	807,709,000	1,050,263,000	0	0
Long-term investments	829,656,000	257,450,000									0	5,509,000	813,637,000	211,960,000	16,019,000	39,981,000
Total assets measured at fair value	1,981,567,000	1,836,047,000									49,949,000	106,387,000	1,915,599,000	1,689,679,000	16,019,000	39,981,000
Liabilities, Fair Value Disclosure	1,022,000	19,100,000									0	0	0	0	1,022,000	19,100,000
Available-for-sale Securities, Other Disclosure Items [Abstract]
Value of auction rate securities for which recent auctions were unsuccessful					$ 9,800,000	$ 9,800,000			$ 5,300,000	$ 18,700,000
Discount rate							2.00%	2.50%
Liquidity risk premium (percentage)							9.10%	29.50%
Anticipated liquidity horizon							7 years	10 years