Mortgage-Backed Securities - By Maturity (Details) - USD ($)	Sep. 30, 2016	Dec. 31, 2015
Mortgage Backed Securities [Abstract]
Greater than one year and less than five years	$ 202,000	$ 0
Greater than five years and less than ten years	280,000	835,000
Greater than or equal to ten years	2,501,015,000	2,157,175,000
Total mortgage-backed securities	$ 2,501,497,000	$ 2,158,010,000