Investment Securities Available for Sale (Summary of Available for Sale Investment Securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 6,998,355	$ 6,647,791
Gross Unrealized Gains	200,890	133,385
Gross Unrealized Losses	(1,752)	(2,506)
Fair Value	7,197,493	6,778,670
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	19,921	19,855
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	19,921
U.S. Government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	168,918	35,499
Gross Unrealized Gains	2,504	1,042
Gross Unrealized Losses	(56)	0
Fair Value	171,366
Mortgage-backed securities issued by U.S. Government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	860,773	56,328
Gross Unrealized Gains	3,721	560
Gross Unrealized Losses	(66)	(72)
Fair Value	864,428
Mortgage-backed securities issued by U.S. Government sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,293,563	5,079,396
Gross Unrealized Gains	150,689	103,495
Gross Unrealized Losses	0	(2,076)
Fair Value	4,444,252
Collateralized mortgage obligations issued by U.S. Government agencies or sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,191,630	629,706
Gross Unrealized Gains	21,597	7,349
Gross Unrealized Losses	(146)	(204)
Fair Value	1,213,081
Commercial mortgage-backed securities issued by U.S. Government agencies or sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	342,974	357,291
Gross Unrealized Gains	22,335	14,301
Gross Unrealized Losses	(56)	0
Fair Value	365,253	371,592
State and municipal securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,003	2,069
Gross Unrealized Gains	4	6
Gross Unrealized Losses	0	0
Fair Value	1,007	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	119,573	144,410
Gross Unrealized Gains	40	2,317
Gross Unrealized Losses	(1,428)	(2)
Fair Value	$ 118,185	$ 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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