Mortgage-Backed Securities Terms of MBS (Details) (Trust Portfolio--MBS Bonds [Member], USD $)				3 Months Ended		12 Months Ended		3 Months Ended		12 Months Ended
	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Standard & Poor's, AA Rating [Member]		Dec. 31, 2012 Standard & Poor's, AA Rating [Member]		Mar. 31, 2013 Standard & Poor's, AAA Rating [Member]		Dec. 31, 2012 Standard & Poor's, AAA Rating [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Instrument, Weighted Average Maturity Date				Jan. 18, 2036				Jan. 14, 2036
Debt, Weighted Average Interest Rate	3.90%			4.00%				4.22%
Debt Instrument, Credit Rating				"AA"	[1]	"AA"	[1]	"AAA"	[1]	"AAA"	[1]
Available for Sale Securities, not Including Premiums	$ 10,000,000	$ 33,665,000	$ 31,600,000	$ 20,990,000				$ 12,675,000

[1] (1) MBS are reported based on the lowest rating issued by a Rating Agency, if more than one rating is issued on the security, at the date presented.