Investments AFS, Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Mar. 31, 2017
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	$ 693,269	$ 384,974
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	(1,371)	(406)
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	693,269	384,974
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	(1,371)	(406)
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	500,561	196,875
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	(1,125)	(227)
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	500,561	196,875
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	(1,125)	(227)
Municipal bonds - tax-exempt
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	0	55,279
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	0	(10)
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	0	55,279
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	0	(10)
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	192,708	132,820
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	(246)	(169)
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	192,708	132,820
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	$ (246)	$ (169)