Investments AFS, Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Dec. 31, 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	$ 1,247,329	$ 384,974
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	(7,156)	(406)
Fair value of available-for-sale securities in a continuous unrealized loss position, greater than twelve months	19,806	0
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, greater than twelve months	(194)	0
Fair value of available-for-sale securities in a continuous unrealized loss position	1,267,135	384,974
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	(7,350)	(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	790,325	196,875
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	(5,615)	(227)
Fair value of available-for-sale securities in a continuous unrealized loss position, greater than twelve months	19,806	0
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, greater than twelve months	(194)	0
Fair value of available-for-sale securities in a continuous unrealized loss position	810,131	196,875
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	(5,809)	(227)
Municipal bonds - taxable
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	9,943
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	(57)
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	9,943
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	(57)
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		55,279
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months		(10)
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		55,279
Accumulated loss, available-for-sale securities in a continuous unrealized loss position		(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	447,061	132,820
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	(1,484)	(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	447,061	132,820
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	$ (1,484)	$ (169)