Other-Than-Temporary Impairment OTTI Credit Losses Recognized During Life of Security (Details) - Residential Mortgage Backed Securities [Member] - Mortgage-backed Securities, Issued by Private Enterprises [Member] - Held-to-maturity Securities [Member] $ in Thousands	Dec. 31, 2019 USD ($)	[1]
Other-than-temporary Impairment Credit Losses Recognized During the Life of the Security [Abstract]
Par Value	$ 565,621
Amortized Cost	386,913
Carrying Value	310,878
Fair Value	473,782
Prime [Member]
Other-than-temporary Impairment Credit Losses Recognized During the Life of the Security [Abstract]
Par Value	3,796
Amortized Cost	3,474
Carrying Value	2,510
Fair Value	3,590
Alt-A [Member]
Other-than-temporary Impairment Credit Losses Recognized During the Life of the Security [Abstract]
Par Value	561,825
Amortized Cost	383,439
Carrying Value	308,368
Fair Value	$ 470,192

[1] Securities are classified based on the classifications of their underlying loan composition at the time of issuance. We instituted and are continuing litigation related to certain of the private-label MBS in which we invested. Our complaint asserts, among others, claims for untrue or misleading statements in the sale of securities. It is possible that classifications of private-label MBS as provided herein when based on classification at the time of issuance as disclosed by those securities' issuance documents, as well as other statements about the securities, are inaccurate.