Mortgage-Backed Securities - By Maturity (Details) - USD ($)	Jun. 30, 2017	Dec. 31, 2016
Mortgage Backed Securities [Abstract]
Greater than one year and less than five years	$ 77,000	$ 157,000
Greater than five years and less than ten years	198,000	277,000
Greater than or equal to ten years	3,737,552,000	3,021,740,000
Total mortgage-backed securities	$ 3,737,827,000	$ 3,022,174,000