Securities (Schedule of Book Values and Fair Values of Investment Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Amortized Cost
Due within one year	$ 0
Due after one year but within five years	28,660
Due after five years but within ten years	74,015
Due after ten years	11,000
Mortgage-backed securities	1,711,809
Amortized Cost	1,825,484	$ 1,432,684
Fair Value
Due within one year	0
Due after one year but within five years	30,023
Due after five years but within ten years	71,578
Due after ten years	10,285
Mortgage-backed securities	1,709,811
Total securities	1,821,697	1,453,132
Amortized Cost
Due within one year	1,587
Due after one year but within five years	537
Due after five years but within ten years	5,355
Due after ten years	164,387
Mortgage-backed securities	26,977
Amortized Cost	198,843	167,551
Fair Value
Due within one year	1,620
Due after one year but within five years	557
Due after five years but within ten years	5,389
Due after ten years	159,859
Mortgage-backed securities	27,999
Total securities	$ 195,424	$ 170,734