Fair Value Measurements Fair Value of Financial Instrument Assumptions (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Fair Value of Financial Instrument Assumptions [Line Items]
Letters of Credit Potential Commitments, Amount	$ 37.7	$ 33.6
First Amendment to New Unsecured Credit Facility [Member]
Fair Value of Financial Instrument Assumptions [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	400.0
Repurchase Agreement [Member]
Fair Value of Financial Instrument Assumptions [Line Items]
Maximum Borrowing Capacity under MIF Warehousing Line	$ 15.0