Commitments and Contingencies (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Acquisition of mortgage loans
Loss Contingencies [Line Items]
Amount of purchase commitment	$ 142.3	$ 602.0
Purchase of limited partnerships and private placement investments
Loss Contingencies [Line Items]
Amount of purchase commitment	$ 473.0	$ 455.9