Investment Securities - Schedule Of Investments Within The Available For Sale Portfolio That Had Unrealized Losses (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Less than 12 months	$ 314,538	$ 960,053
12 months or longer	642,321	3,112,632
Total fair value	956,859	4,072,685
Unrealized Losses
Less than 12 months	(925)	(15,826)
12 months or longer	(7,631)	(92,005)
Total unrealized losses	(8,556)	(107,831)
U.S. treasuries
Schedule of Investments [Line Items]
Less than 12 months	0	0
12 months or longer	100	98
Total fair value	100	98
Unrealized Losses
Less than 12 months	0	0
12 months or longer	0	(2)
Total unrealized losses	0	(2)
Government agency issued MBS
Schedule of Investments [Line Items]
Less than 12 months	78,633	597,008
12 months or longer	219,235	1,537,106
Total fair value	297,868	2,134,114
Unrealized Losses
Less than 12 months	(222)	(12,335)
12 months or longer	(2,523)	(46,065)
Total unrealized losses	(2,745)	(58,400)
Government agency issued CMO
Schedule of Investments [Line Items]
Less than 12 months	234,437	290,863
12 months or longer	397,826	1,560,420
Total fair value	632,263	1,851,283
Unrealized Losses
Less than 12 months	(681)	(2,860)
12 months or longer	(5,018)	(45,821)
Total unrealized losses	(5,699)	(48,681)
Other U.S. government agencies
Schedule of Investments [Line Items]
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
Schedule of Investments [Line Items]
Less than 12 months	0	25,114
12 months or longer	25,160	15,008
Total fair value	25,160	40,122
Unrealized Losses
Less than 12 months	0	(344)
12 months or longer	(90)	(117)
Total unrealized losses	(90)	(461)
States and municipalities
Schedule of Investments [Line Items]
Less than 12 months	1,468	17,292
12 months or longer	0	0
Total fair value	1,468	17,292
Unrealized Losses
Less than 12 months	(22)	(214)
12 months or longer	0	0
Total unrealized losses	$ (22)	$ (214)