Summary of Investments - SOI Mortgage loans on real estate by component (Details 3) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Principal	$ 3,936,617	$ 3,558,863
Unamortized premium (discount) and fees, net	4,244	5,541
Foreign exchange translation	3,000	(2,696)
Mortgage provision allowance	(773)	(2,882)	$ (2,890)
Total mortgage loans	$ 3,943,088	$ 3,558,826