Fair Value Measurements Fair Value of Financial Instrument Assumptions (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Fair Value of Financial Instrument Assumptions [Line Items]
Letters of Credit Potential Commitments, Amount	$ 36.4	$ 42.5
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
Warehousing Agreement - Second Amended and Restated [Member]
Fair Value of Financial Instrument Assumptions [Line Items]
Maximum Borrowing Capacity under MIF Warehousing Line	125.0
Repurchase Agreement [Member]
Fair Value of Financial Instrument Assumptions [Line Items]
Maximum Borrowing Capacity under MIF Warehousing Line	$ 15.0