Investment Securities - Schedule Of Investments Within The Available For Sale Portfolio That Had Unrealized Losses (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Less than 12 months	$ 1,541,140	$ 655,820
12 months or longer	0	553,979
Total fair value	1,541,140	1,209,799
Unrealized Losses
Less than 12 months	(3,610)	(3,616)
12 months or longer	0	(7,288)
Total unrealized losses	(3,610)	(10,904)
U.S. treasuries
Schedule of Investments [Line Items]
Less than 12 months	299,934	0
12 months or longer	0	100
Total fair value	299,934	100
Unrealized Losses
Less than 12 months	(3)	0
12 months or longer	0	0
Total unrealized losses	(3)	0
Government agency issued MBS
Schedule of Investments [Line Items]
Less than 12 months	568,733	174,983
12 months or longer	0	192,755
Total fair value	568,733	367,738
Unrealized Losses
Less than 12 months	(2,158)	(495)
12 months or longer	0	(2,061)
Total unrealized losses	(2,158)	(2,556)
Government agency issued CMO
Schedule of Investments [Line Items]
Less than 12 months	636,386	378,815
12 months or longer	0	361,124
Total fair value	636,386	739,939
Unrealized Losses
Less than 12 months	(1,439)	(1,970)
12 months or longer	0	(5,227)
Total unrealized losses	(1,439)	(7,197)
Other U.S. government agencies
Schedule of Investments [Line Items]
Less than 12 months	30,896	98,471
12 months or longer	0	0
Total fair value	30,896	98,471
Unrealized Losses
Less than 12 months	(4)	(1,121)
12 months or longer	0	0
Total unrealized losses	(4)	(1,121)
States and municipalities
Schedule of Investments [Line Items]
Less than 12 months	5,191	3,551
12 months or longer	0	0
Total fair value	5,191	3,551
Unrealized Losses
Less than 12 months	(6)	(30)
12 months or longer	0	0
Total unrealized losses	$ (6)	$ (30)