Fair Value Measurements Fair Value of Financial Instrument Assumptions (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Fair Value of Financial Instrument Assumptions [Line Items]
Letters of Credit Potential Commitments, Amount	$ 41.4	$ 37.7
Second Amendment to Credit Facility [Member]
Fair Value of Financial Instrument Assumptions [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	475.0
Warehousing Agreement - First Amendment to Second Amended and Restated [Member]
Fair Value of Financial Instrument Assumptions [Line Items]
M/I Financial Maximum Borrowing Capacity	125.0
M/I Financial Temporary Increase Maximum Borrowing Capacity	150.0
Repurchase Agreement - Amendment No. 4 [Member]
Fair Value of Financial Instrument Assumptions [Line Items]
M/I Financial Maximum Borrowing Capacity	$ 35.0