Investments Investments AFS, Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Jun. 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	$ 904,461	$ 339,361
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	(2,914)	(370)
Fair value of available-for-sale securities in a continuous unrealized loss position, greater than twelve months	42,970	48,963
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, greater than twelve months	(172)	(104)
Fair value of available-for-sale securities in a continuous unrealized loss position	947,431	388,324
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	(3,086)	(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	442,847	162,948
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	(1,061)	(142)
Fair value of available-for-sale securities in a continuous unrealized loss position, greater than twelve months	14,911	29,942
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, greater than twelve months	(89)	(58)
Fair value of available-for-sale securities in a continuous unrealized loss position	457,758	192,890
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	(1,150)	(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	37,104	13,318
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	(765)	(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	37,104	13,318
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	(765)	(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	424,510	163,095
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	(1,088)	(219)
Fair value of available-for-sale securities in a continuous unrealized loss position, greater than twelve months	28,059	19,021
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, greater than twelve months	(83)	(46)
Fair value of available-for-sale securities in a continuous unrealized loss position	452,569	182,116
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	$ (1,171)	$ (265)