Note Debt Securities Available for Sale (Name of issuers and the aggregate amortized cost and fair value of the debt securities of such issuer, in which the aggregate amortized cost exceeds 10% of stockholders' equity) (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Investment Holdings [Line Items]
Amortized Cost	$ 13,611,361	$ 13,478,562
Fair Value	13,542,695	13,300,184
FNMA
Investment Holdings [Line Items]
Amortized Cost	3,273,409	2,999,110
Fair Value	3,220,000	2,901,904
Freddie Mac
Investment Holdings [Line Items]
Amortized Cost	1,312,120	1,095,855
Fair Value	$ 1,293,207	$ 1,058,013