Subsequent Events (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Feb. 03, 2015
Subsequent Event [Line Items]
Long-term Debt, Gross	$ 4,195.8	$ 3,838.5
Subsequent Event [Member] | Recourse Fixed Rate Debt Unsecured Twenty One [Member]
Subsequent Event [Line Items]
Long-term Debt, Gross			100.0
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate			2.60%
Subsequent Event [Member] | Recourse Fixed Rate Debt Unsecured Twenty Two [Member]
Subsequent Event [Line Items]
Long-term Debt, Gross			300.0
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate			3.25%
Subsequent Event [Member] | Recourse Fixed Rate Debt Unsecured Twenty Three [Member]
Subsequent Event [Line Items]
Long-term Debt, Gross			$ 250.0
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate			4.50%