Investments Continuous Unrealized Loss Position (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Fair value of available-for-sale securities in a continuous unrealized loss position, less than twelve months	$ 339,361	$ 781,501
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	(370)	(3,989)
Fair value of available-for-sale securities in a continuous unrealized loss position, greater than twelve months	48,963	1,196
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, greater than twelve months	(104)	(1)
Fair value of available-for-sale securities in a continuous unrealized loss position	388,324	782,697
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	(474)	(3,990)
Government agency bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value of available-for-sale securities in a continuous unrealized loss position, less than twelve months	162,948	522,159
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	(142)	(3,171)
Fair value of available-for-sale securities in a continuous unrealized loss position, greater than twelve months	29,942	0
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, greater than twelve months	(58)	0
Fair value of available-for-sale securities in a continuous unrealized loss position	192,890	522,159
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	(200)	(3,171)
Municipal bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value of available-for-sale securities in a continuous unrealized loss position, less than twelve months	13,318	2,625
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	(9)	(13)
Fair value of available-for-sale securities in a continuous unrealized loss position, greater than twelve months	0	1,196
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, greater than twelve months	0	(1)
Fair value of available-for-sale securities in a continuous unrealized loss position	13,318	3,821
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	(9)	(14)
Corporate bonds and debt [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value of available-for-sale securities in a continuous unrealized loss position, less than twelve months	163,095	256,717
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	(219)	(805)
Fair value of available-for-sale securities in a continuous unrealized loss position, greater than twelve months	19,021	0
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, greater than twelve months	(46)	0
Fair value of available-for-sale securities in a continuous unrealized loss position	182,116	256,717
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	$ (265)	$ (805)