MBS, CRT Securities and MSR Related Assets (Sale of MBS) (Details) - Non-Agency MBS - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sale of MBS categorized as available-for-sale	$ 104.0	$ 85.6	$ 70.7
Gains on sales of MBS	$ 39.9	$ 35.8	$ 34.9