AFS, Debt Maturities (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	$ 1,677,422	$ 1,301,989
Available-for-sale, gross unrealized gains	3,462	6,189
Available-for-sale, gross unrealized losses	(3,990)	(465)
Available-for-sale, estimated fair value	1,676,894	1,307,713
Due in one year or less [Member]
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	210,129	350,349
Available-for-sale, gross unrealized gains	1,234	1,933
Available-for-sale, gross unrealized losses	0	(3)
Available-for-sale, estimated fair value	211,363	352,279
Due after one year and through five years [Member]
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	1,308,844	795,952
Available-for-sale, gross unrealized gains	2,228	3,666
Available-for-sale, gross unrealized losses	(1,958)	(212)
Available-for-sale, estimated fair value	1,309,114	799,406
Due after five years and through ten years [Member]
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	142,434	115,901
Available-for-sale, gross unrealized gains	0	116
Available-for-sale, gross unrealized losses	(2,032)	(250)
Available-for-sale, estimated fair value	140,402	115,767
Due after ten years [Member]
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	9,825	28,327
Available-for-sale, gross unrealized gains	0	235
Available-for-sale, gross unrealized losses	0	0
Available-for-sale, estimated fair value	9,825	28,562
Total Maturities Member
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	1,671,232	1,290,529
Available-for-sale, gross unrealized gains	3,462	5,950
Available-for-sale, gross unrealized losses	(3,990)	(465)
Available-for-sale, estimated fair value	1,670,704	1,296,014
Equity Securities [Member]
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost		5,270
Available-for-sale, gross unrealized gains		239
Available-for-sale, gross unrealized losses		0
Available-for-sale, estimated fair value		$ 5,509