Summary of Investments - SOI Mortgage loans on real estate by component (Details 3) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Principal	$ 3,565,454	$ 3,242,627
Unamortized premium (discount) and fees, net	6,530	7,967
Foreign exchange translation	(988)	0
Mortgage provision allowance	(2,882)	(2,890)	$ (2,890)
Total mortgage loans	$ 3,568,114	$ 3,247,704