Loans and Investments, Interest Reserves (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) loan	Dec. 31, 2017 USD ($) loan
Loans and Investments
Interest reserve held	$ 46.3	$ 52.5
Number of loans covered under interest reserve | loan	84	81
Aggregate UPB covered under interest reserve	$ 1,660.0	$ 1,570.0