Investment Securities Available for Sale (Summary of Available for Sale Investment Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 7,395,265	$ 6,647,791
Gross Unrealized Gains	182,653	133,385
Gross Unrealized Losses	(11,393)	(2,506)
Fair Value	7,566,525	6,778,670
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	20,254	19,855
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	20,254	19,855
U.S. Government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	153,984	35,499
Gross Unrealized Gains	3,056	1,042
Gross Unrealized Losses	(426)	0
Fair Value	156,614	36,541
Mortgage-backed securities issued by U.S. Government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,132,816	56,328
Gross Unrealized Gains	2,655	560
Gross Unrealized Losses	(6,079)	(72)
Fair Value	1,129,392	56,816
Mortgage-backed securities issued by U.S. Government sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,402,051	5,079,396
Gross Unrealized Gains	138,611	103,495
Gross Unrealized Losses	(276)	(2,076)
Fair Value	4,540,386	5,180,815
Collateralized mortgage obligations issued by U.S. Government agencies or sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,331,011	629,706
Gross Unrealized Gains	18,495	7,349
Gross Unrealized Losses	(4,222)	(204)
Fair Value	1,345,284	636,851
Commercial mortgage-backed securities issued by U.S. Government agencies or sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	334,463	357,291
Gross Unrealized Gains	19,694	14,301
Gross Unrealized Losses	0	0
Fair Value	354,157	371,592
State and municipal securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	500	2,069
Gross Unrealized Gains	1	6
Gross Unrealized Losses	0	0
Fair Value	501	2,075
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		323,237
Gross Unrealized Gains		4,315
Gross Unrealized Losses		(152)
Fair Value		327,400
Corporate debt securities and other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	20,186	144,410
Gross Unrealized Gains	141	2,317
Gross Unrealized Losses	(390)	(2)
Fair Value	$ 19,937	$ 146,725

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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