AFS, Debt Maturities (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Sep. 30, 2014	Mar. 31, 2014
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	$ 1,643,964	$ 1,677,422
Available-for-sale, gross unrealized gains	2,345	3,462
Available-for-sale, gross unrealized losses	(3,362)	(3,990)
Available-for-sale, estimated fair value	1,642,947	1,676,894
Due in one year or less [Member]
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	250,615
Available-for-sale, gross unrealized gains	742
Available-for-sale, gross unrealized losses	(5)
Available-for-sale, estimated fair value	251,352
Due after one year and through five years [Member]
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	1,264,898
Available-for-sale, gross unrealized gains	1,552
Available-for-sale, gross unrealized losses	(2,227)
Available-for-sale, estimated fair value	1,264,223
Due after five years and through ten years [Member]
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	112,436
Available-for-sale, gross unrealized gains	51
Available-for-sale, gross unrealized losses	(1,130)
Available-for-sale, estimated fair value	111,357
Due after ten years [Member]
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	9,825
Available-for-sale, gross unrealized gains	0
Available-for-sale, gross unrealized losses	0
Available-for-sale, estimated fair value	9,825
Total Maturities [Member]
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	1,637,774
Available-for-sale, gross unrealized gains	2,345
Available-for-sale, gross unrealized losses	(3,362)
Available-for-sale, estimated fair value	$ 1,636,757