Comprehensive Income (Details) (USD $)	3 Months Ended		6 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Comprehensive Income [Abstract]
Comprehensive Income, Net of Tax, Attributable to Parent	$ 73,400,000	$ 109,100,000	$ 174,600,000	$ 191,400,000
Change in unrealized gains and losses on investments, net of tax effect of $(547), $1,265, $(428) and $(3,252), respectively.	(5,881,000)	6,043,000	(3,954,000)	(1,242,000)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax	$ (547,000)	$ 1,265,000	$ (428,000)	$ (3,252,000)