AFS, Debt Maturities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Jun. 30, 2014	Mar. 31, 2014
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	$ 1,667,120	$ 1,677,422
Available-for-sale, gross unrealized gains	3,994	3,462
Available-for-sale, gross unrealized losses	(1,484)	(3,990)
Available-for-sale, estimated fair value	1,669,630	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	199,349
Available-for-sale, gross unrealized gains	841
Available-for-sale, gross unrealized losses	(6)
Available-for-sale, estimated fair value	200,184
Due after one year and through five years [Member]
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	1,324,320
Available-for-sale, gross unrealized gains	3,100
Available-for-sale, gross unrealized losses	(762)
Available-for-sale, estimated fair value	1,326,658
Due after five years and through ten years [Member]
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	127,436
Available-for-sale, gross unrealized gains	53
Available-for-sale, gross unrealized losses	(716)
Available-for-sale, estimated fair value	126,773
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	1,660,930
Available-for-sale, gross unrealized gains	3,994
Available-for-sale, gross unrealized losses	(1,484)
Available-for-sale, estimated fair value	$ 1,663,440