Derivative Financial Instruments - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Gross gain (loss) associated with forward sales commitments and IRLCs	$ (2.6)	$ 2.7	$ 2.0