Fair Value Measurements Fair Value of Financial Instrument Assumptions (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Fair Value of Financial Instrument Assumptions [Line Items]
Letters of Credit Potential Commitments, Amount	$ 35.0	$ 33.6
First Amendment to New Unsecured Credit Facility [Member]
Fair Value of Financial Instrument Assumptions [Line Items]
Line of Credit Facility, Current Borrowing Capacity	300
Warehousing Agreement - First Amendment to Amended and Restated [Member]
Fair Value of Financial Instrument Assumptions [Line Items]
Maximum Borrowing Capacity under MIF Warehousing Line	110.0
Repurchase Agreement [Member]
Fair Value of Financial Instrument Assumptions [Line Items]
Maximum Borrowing Capacity under MIF Warehousing Line	$ 15.0