Investments Investments AFS, Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Mar. 31, 2015
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	$ 566,341	$ 339,361
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	(1,928)	(370)
Fair value of available-for-sale securities in a continuous unrealized loss position, greater than twelve months	36,828	48,963
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, greater than twelve months	(69)	(104)
Fair value of available-for-sale securities in a continuous unrealized loss position	603,169	388,324
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	(1,997)	(474)
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	140,775	162,948
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	(51)	(142)
Fair value of available-for-sale securities in a continuous unrealized loss position, greater than twelve months	10,000	29,942
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, greater than twelve months	0	(58)
Fair value of available-for-sale securities in a continuous unrealized loss position	150,775	192,890
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	(51)	(200)
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	34,222	13,318
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	(516)	(9)
Fair value of available-for-sale securities in a continuous unrealized loss position, greater than twelve months	0	0
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, greater than twelve months	0	0
Fair value of available-for-sale securities in a continuous unrealized loss position	34,222	13,318
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	(516)	(9)
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	391,344	163,095
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	(1,361)	(219)
Fair value of available-for-sale securities in a continuous unrealized loss position, greater than twelve months	26,828	19,021
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, greater than twelve months	(69)	(46)
Fair value of available-for-sale securities in a continuous unrealized loss position	418,172	182,116
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	$ (1,430)	$ (265)