Investments in Tax-Exempt Bonds Effective earning (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended							6 Months Ended
	Jun. 30, 2012	Jun. 30, 2012 Effective rate - maximum [Member]	Dec. 31, 2011 Effective rate - maximum [Member]	Jun. 30, 2012 Effective rate - minimum [Member]	Dec. 31, 2011 Effective rate - minimum [Member]	Jun. 30, 2012 10% adverse [Member] Effective rate - maximum [Member]	Jun. 30, 2012 10% adverse [Member] Effective rate - minimum [Member]	Jun. 30, 2012 Minimum [Member] Bella Vista, Woodland Park and Woodlynn Village [Member]
Schedule of Available-for-sale Securities [Line Items]
Adverse change rate	10.00%
Effective rate		8.60%	9.00%	5.70%	6.30%	9.40%	6.30%
10% unrealized loss	$ 10.2
Unrealized Loss Position for Available-for-sale Securities								12 months