Securities (Schedule of Book Values and Fair Values of Investment Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Amortized Cost
Due within one year	$ 1,001
Due after one year but within five years	27,639
Due after five years but within ten years	84,014
Due after ten years	1,000
Mortgage-backed securities	2,187,209
Amortized Cost	2,300,863	$ 1,432,684
Fair Value
Due within one year	1,031
Due after one year but within five years	28,801
Due after five years but within ten years	82,739
Due after ten years	974
Mortgage-backed securities	2,191,530
Total securities	2,305,075	1,453,132
Amortized Cost
Due within one year	1,271
Due after one year but within five years	536
Due after five years but within ten years	8,388
Due after ten years	335,670
Mortgage-backed securities	22,028
Amortized Cost	367,893	167,551
Fair Value
Due within one year	1,282
Due after one year but within five years	550
Due after five years but within ten years	8,525
Due after ten years	336,710
Mortgage-backed securities	22,886
Total securities	$ 369,953	$ 170,734