Investments Available-for-Sale Securities by Contractual Maturity (Details) - USD ($) $ in Thousands		Mar. 31, 2020	Dec. 31, 2019
Available-for-sale Securities, Debt Maturities [Abstract]
Amortized Cost	[1]	$ 7,405,993	$ 7,482,922
Fair Value		7,246,334	7,409,000
Other Than MBS [Member]
Available-for-sale Securities, Debt Maturities [Abstract]
Due in one year or less - Amortized Cost		7,600	7,600
Due in one year or less - Fair Value		7,600	7,563
Due after one year through five years - Amortized Cost		61,720	61,720
Due after one year through five years - Fair Value		57,795	57,089
Due after five years through 10 years - Amortized Cost		509,594	477,232
Due after five years through 10 years - Fair Value		488,131	463,465
Due after 10 years - Amortized Cost		473,906	406,249
Due after 10 years - Fair Value		398,666	373,511
Amortized Cost	[1]	1,052,820	952,801
Fair Value		952,192	901,628
MBS [Member]
Available-for-sale Securities, Debt Maturities [Abstract]
Amortized Cost	[1]	6,353,173	6,530,121
Fair Value		6,294,142	6,507,372
Amortized Cost - MBS	[2]	6,353,173	6,530,121
Fair Value - MBS	[2]	$ 6,294,142	$ 6,507,372

[1] Amortized cost of available-for-sale securities includes adjustments made to the cost basis of an investment for accretion, amortization, collection of cash, and fair-value hedge accounting adjustments. Amortized cost excludes accrued interest receivable of $20.0 million and $27.5 million at March 31, 2020, and December 31, 2019.

[2] MBS are not presented by contractual maturity because their expected maturities will likely differ from contractual maturities because borrowers of the underlying loans may have the right to call or prepay obligations with or without call or prepayment fees.