Investments AFS, Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Mar. 31, 2016
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	$ 30,007
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	(114)
Fair value of available-for-sale securities in a continuous unrealized loss position, greater than twelve months	0
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, greater than twelve months	0
Fair value of available-for-sale securities in a continuous unrealized loss position	30,007
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	(114)
Government agency bonds
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	19,912	$ 148,562
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	(88)	(99)
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	19,912	148,562
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	(88)	$ (99)
Corporate bonds and debt
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	10,095
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	(26)
Fair value of available-for-sale securities in a continuous unrealized loss position, greater than twelve months	0
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, greater than twelve months	0
Fair value of available-for-sale securities in a continuous unrealized loss position	10,095
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	$ (26)