Investment Securities Available for Sale (Summary of Available for Sale Investment Securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 6,656,754	$ 6,647,791
Gross Unrealized Gains	294,334	133,385
Gross Unrealized Losses	(13,848)	(2,506)
Fair Value	6,937,240	6,778,670
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	19,918	19,855
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	19,918	19,855
U.S. Government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	72,424	35,499
Gross Unrealized Gains	2,469	1,042
Gross Unrealized Losses	(37)	0
Fair Value	74,856	36,541
Mortgage-backed securities issued by U.S. Government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	53,557	56,328
Gross Unrealized Gains	1,866	560
Gross Unrealized Losses	(1)	(72)
Fair Value	55,422	56,816
Mortgage-backed securities issued by U.S. Government sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,312,698	5,079,396
Gross Unrealized Gains	237,127	103,495
Gross Unrealized Losses	0	(2,076)
Fair Value	5,549,825	5,180,815
Collateralized mortgage obligations issued by U.S. Government agencies or sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	601,585	629,706
Gross Unrealized Gains	27,297	7,349
Gross Unrealized Losses	(2)	(204)
Fair Value	628,880	636,851
Commercial mortgage-backed securities issued by U.S. Government agencies or sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	311,235	357,291
Gross Unrealized Gains	24,502	14,301
Gross Unrealized Losses	0	0
Fair Value	335,737	371,592
State and municipal securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,007	2,069
Gross Unrealized Gains	2	6
Gross Unrealized Losses	(4)	0
Fair Value	1,005	2,075
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	154,954	323,237
Gross Unrealized Gains	150	4,315
Gross Unrealized Losses	(9,129)	(152)
Fair Value	145,975	327,400
Corporate debt securities and other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	129,376	144,410
Gross Unrealized Gains	921	2,317
Gross Unrealized Losses	(4,675)	(2)
Fair Value	$ 125,622	$ 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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Amount, before tax, of unrealized loss 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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Amortized cost of 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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