Investments in Tax-Exempt Bonds Effective earning (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended							3 Months Ended
	Mar. 31, 2013	Mar. 31, 2013 Effective rate - maximum [Member]	Dec. 31, 2012 Effective rate - maximum [Member]	Mar. 31, 2013 Effective rate - minimum [Member]	Dec. 31, 2012 Effective rate - minimum [Member]	Mar. 31, 2013 10% adverse [Member] Effective rate - maximum [Member]	Mar. 31, 2013 10% adverse [Member] Effective rate - minimum [Member]	Mar. 31, 2013 Minimum [Member] Woodland Park [Member]
Schedule of Available-for-sale Securities [Line Items]
Adverse change rate	10.00%
Effective rate		8.30%	8.40%	5.50%	5.70%	9.10%	6.00%
10% unrealized loss	$ 15.5
Unrealized Loss Position for Available-for-sale Securities								12 months