Investment Securities - Schedule Of Investments Within The Available For Sale Portfolio That Had Unrealized Losses (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value
Less than 12 months	$ 2,405	$ 960,053
12 months or longer	3,688,411	3,112,632
Total fair value	3,690,816	4,072,685
Unrealized Losses
Less than 12 months	(13)	(15,826)
12 months or longer	(50,497)	(92,005)
Total unrealized losses	(50,510)	(107,831)
U.S. treasuries
Fair Value
Less than 12 months	0	0
12 months or longer	99	98
Total fair value	99	98
Unrealized Losses
Less than 12 months	0	0
12 months or longer	(1)	(2)
Total unrealized losses	(1)	(2)
Government agency issued MBS
Fair Value
Less than 12 months	1,542	597,008
12 months or longer	1,948,718	1,537,106
Total fair value	1,950,260	2,134,114
Unrealized Losses
Less than 12 months	(13)	(12,335)
12 months or longer	(21,347)	(46,065)
Total unrealized losses	(21,360)	(58,400)
Government agency issued CMO
Fair Value
Less than 12 months	863	290,863
12 months or longer	1,699,529	1,560,420
Total fair value	1,700,392	1,851,283
Unrealized Losses
Less than 12 months	0	(2,860)
12 months or longer	(28,765)	(45,821)
Total unrealized losses	(28,765)	(48,681)
Other U.S. government agencies
Fair Value
Less than 12 months		29,776
12 months or longer		0
Total fair value		29,776
Unrealized Losses
Less than 12 months		(73)
12 months or longer		0
Total unrealized losses		(73)
Corporates and other debt
Fair Value
Less than 12 months	0	25,114
12 months or longer	40,065	15,008
Total fair value	40,065	40,122
Unrealized Losses
Less than 12 months	0	(344)
12 months or longer	(384)	(117)
Total unrealized losses	$ (384)	(461)
States and municipalities
Fair Value
Less than 12 months		17,292
12 months or longer		0
Total fair value		17,292
Unrealized Losses
Less than 12 months		(214)
12 months or longer		0
Total unrealized losses		$ (214)