Mortgage-Backed Securities - By Maturity (Details) - USD ($)	Jun. 30, 2015	Dec. 31, 2014
Mortgage Backed Securities [Abstract]
Greater than five years and less than ten years	$ 697,000	$ 967,000
Greater than or equal to ten years	2,174,174,000	1,548,204,000
Total mortgage-backed securities	$ 2,174,871,000	$ 1,549,171,000